I suspect your premise about gaining more points for winning than losing is slightly off. Generally speaking, once your point level stabilizes due to finding your actual skill level, your points will remain the same relative to others playing the game if you don't get any better/worse.
However, the total number of points everyone has will trend upward due to the Bonus Pool. This does mean it rewards playing more often, but only to a certain extent in a certain time period (a few games per week, I suspect). Once you've exhausted your bonus pool, your ranking points will again more-or-less stabilize based on your skill.
One of the reasons this was done was to encourage people to actually play the game. Since everyone's points are constantly going up, you have to play to keep up with your peers. If this artificial inflation of points due to the bonus pool wasn't in place, the player at the very top would - in theory - not have much reason to play; he's the best and his points will stay the highest until someone usurps him. This way, he has to keep playing to maintain his status. As with any fair matchmaking system, the more people playing, the better, and I think Blizzard is trying to encourage more people to play.
However, none of this is cut and dry so there's no one reason for any of these decisions; likewise it's not at all obvious whether this system is superior to the traditional ranking system you mention.
That said, here are some good articles on the subject:
Note also that the points you see displayed and used for your ranking may be separate from the underlying statistic used for matching you up against other players. The two stats should converge on relatively the same thing, but the Bonus Pool adds the extra incentive to play more often.
Divisions are just a segment of a League, they don't really mean anything at all. You will play with people from many different divisions while you are playing.
When you qualify for a league, you are randomly placed into whatever division is currently being filled.
All the divisions do is break the people up into groups of around 100, and give you a rank to strive for. You shouldn't ever change division until you change league. And your league will change as you do well or poorly in your current league.
They poll your skill randomly and decide weather or not to bump you one way or the other.
You can on occasion have matches against people from other leagues, to test your skill level.
Lots more info here: http://starcraft.wikia.com/wiki/League_%28StarCraft_II%29
Best Answer
It is possible to reach an unlimited value of points and still remain in bronze league. Blizzard's ranking system will randomly 'test' you when it feels you might be ready to move up to a different league. It will put you against a player from the next league and check to see if your 'ready' to move up. To answer your existing question, these players are winning against players they are expected to win against, but losing against players in the next league.
Blizzard views this as a perfect balance. As long as they continue to win, they will continue to gain more points. But as long as the continue to lose their 'test' matches they will remain in that league. Keep in mind that being 1st in a division doesn't mean you need to be in a different league, someone has to be in first.
Both of these team liquid threads give some great information about SC2 ladder and how to move between leagues
SC2 Ladder Analysis: What YOU Need to Know
SC2 Ladder Analysis: Part 2
Hope these help