This lack of respect for the language of origin not a phenomenon unique to English. When a word is borrowed into one language from another, unexpected things can happen.

I would argue that, for many examples you've given in your question, the actual perception of a singular-plural relationship is messy in practice, and the application of the plural is inconsistent.

Data: Using data as a collective noun with singular agreement is more common than using it with plural agreement. More in another thread from this site.

Alumni: I have heard as many people also use alumni for the singular, or even alum, as I have heard use alumnus for the singular. I imagine my experience with this word is typical (at least in the US), though certainly not universal. In any case, it is messy.

Media: The words media and medium don't even seem to correspond in any meaningful way in actual English usage. The word media has forked off and become a different word entirely. The word media is clearly used as a collective singular noun, as shown in newer constructions like multimedia (not multimedium even though we don't say multistages, multicores, multicycles, multistories, etc.). You will find few people who will ever say "Mass Medium". We talk about someone having "media savvy" even though we wouldn't say "computers savvy" (even though they can work with more than one computer). This is because, in English, these sorts of constructions always use the singular noun, whether it is collective or not. The way that media is used is evidence of how the word is actually parsed, perceived, and used by English speakers.

Another example of how foreign language morphology often doesn't mesh well: people try to pluralize octopus and virus as octopi and viri/virii, respectively. Virus was a mass noun in Latin, where we got the word. The word octopus comes from Greek and would take the plural form octopodes in Greek.

My main point is this: there is only a weak, inconsistent application of this -us to -a or -us to -i to begin with. So forums (like statuses and others) is a word even though we also sometimes have this other rule. Our language seems to continually push us towards either dropping the foreign pluralization in some way or another, or reanalyzing the plural as another distinct word. So I see this confusion as the language trying to mash these words around to make them fit our language naturally.

If we hadn't become so darn literate and knowledgeable in the past few centuries, I imagine these plurals would have regularized by now :)

Etymonline to the rescue:

syllabus (n.)

1650s, “table of contents of a series of lectures, etc.,” from Late Latin syllabus “list,” ultimately a misreading of Greek sittybos “parchment label, table of contents,” of unknown origin. The misprint appeared in a 15c. edition of Cicero’s Ad Atticum (see OED). Had it been a real word, the proper plural would be syllabi.

The full OED entry for the word mentioned by Etymonline is much too long to fully cite here, but includes the following material pertinent to the current discussion:

syllabus (n.)

Etymology

< modern Latin syllabus, usually referred to an alleged Greek σύλλαβος. Syllabus appears to be founded on a corrupt reading syllabos in some early printed editions — the Medicean MS. has sillabos — of Cicero Epp. ad Atticum ɪ. iv, where the reading indicated as correct by comparison with the MS. readings in ɪ. v. and viii. is sittybas or Greek σιττύβας, accusative plural of sittyba, σιττύβα parchment label or title-slip on a book. (Compare Tyrrell and Purser Correspondence of Cicero nos. 107, 108, 112, Comm. and Adnot. Crit.) Syllabos was græcized by later editors as συλλάβους, from which a spurious σύλλαβος was deduced and treated as a derivative of συλλαμβάνειν to put together, collect (compare ꜱʏʟʟᴀʙʟᴇ n.).

In the passage from S. Augustine’s Confessions xɪɪɪ. xv. (‘ibi legunt [sc. angeli] sine syllabis temporum quid velit aeterna voluntas tua’) commonly adduced as further evidence of Latin syllabus, the word is clearly syllaba syllable.

The earliest provided citation dates from a 1656 work by antiquary and lexicographer Thomas Blount in his Glossographia; or, A dictionary interpreting all such hard words, whether Hebrew, Greek or Latin... as are now used in our refined English tongue (1ˢᵗ edition, 1656, London), where he writes:

• Syllabus, a Table or Index in a Book, to shew places or matter by Letters or Figures.

This was quickly followed in 1667 by Jeremy Taylor, Church of Ireland Bishop of Down and Connor, in the 4ᵗʰ edition of his work The great exemplar of sanctity and holy life (1ˢᵗ edition 1649, 4ᵗʰ edition 1667), where he writes at ɪ. vi. §22. 160:

• The Apostle expresses it still by Synonyma’s, Tasting of the heavenly gift, and made partakers of the holy Ghost..; all which also are a syllabus or collection of the several effects of the graces bestowed in Baptism.

So, it seems that it is based on a misspelling of a Greek word — but that would hardly give one reason to form the plural in Greek starting from the Latin(ized) form.

According to both Merriam-Webster and the OED alike, the plural can be either syllabi /ˈsɪləbaɪ/ or syllabuses /ˈsɪləbəsɪz/, but with Etymonline’s “no real word” verdict, I’d go for the latter.