Electrical – Why is this switch box wired like a 3-way when there’s no other switch

All the ground wires in a box should be connected together, so add a pigtail from each switch to the ground wire nut, and connect them all together. If this gets to be a lot of wires (sounds like 3 cables and 2 switches, so 5 total) it may be easier to use a push-in connector instead:

or for 6 conductors:

It's tapped like that because that's the way it was supposed to be done when that work was done. Back in those days (<2011) if you had a single 2-wire feed (romex) to and fro a switch box. The white wire would, according to more recent codes prior to NEC 2011, and possibly then too but not followed, have been taped a solid color other than green to notify you that it is indeed hot.

This is done to ensure the wire feeding power to the light is black as it would/should be in normal situations; it's safety and economy. The other way to deal with this is to pull a 3-wire feed to the switch. Today this is required not to rid the world of the marking style, as it is still permitted under certain exceptions today, but to cover certain types of switches which require it. As recent as NEC 2008 it was not mandatory unless the manufacturer of the device required it.

You should tape it up (the white wire), then replace all those taped joints with wire nuts appropriately sized for the taps.