The best way I've thought of to explain it is like this:
When you buy into DVC, you're "buying" a teeny tiny percentage of your home resort (and "buying" is a bit misleading, since eventually the deed reverts back to Disney). It's just that your teeny tiny percentage is represented to you in the form of points. For the people who run DVC to just increase point values across the board would be like saying "Yeah, I know you have a contract that says you own THIS much of the Saratoga Springs resort, but really? As of now, you only own THAT much. Sucker."
Imagine if a buddy of yours wanted to buy a pizza place and needed a minor partner, so you bought into the place, 10% of the restaurant. And at the end of the year, when he divvies up profits, he only gives you profits for 5% ownership and says "yeah, I decided you only own 5% now." Well, THAT'S illegal, in'nt it? And it would be the same if DVC just increased point values across the board.
As others have been said, they can reconfigure the point values, adding points here while reducing values there so it all balances out at years' end. And even that would only be to spread out users more evenly. When DVC first started, lots of people were using their points for weekend jaunts. So points were adjusted so that Friday and Saturday night stays used much more of your points than the rest of the week. Then, DVC had the opposite problem, as people were maximizing the use of their points by taking a few Sunday-Friday trips over the course of a year. So they're further finessing point values so weekends use more points, but not as much as they used to. Trying to jigger it so that more people use their points in weekly increments. The only negative is for people who bought into DVC because the point value structure, at the time they bought, made more sense to them than it does now. I suspect most people arrange to go for 7 or more nights, as opposed to just weekends or just arrive-Sunday-depart-Friday, so those people aren't affected as much.