Generating culture and building units are different. The more units you build, the more it will cost you, so the increasing number is also an increasing cost and an empire that gets difficult to maintain. Whatsmore, if you build enough units to fill up the entire map, you can still choose to disband the new one, keeping the current situation unchanged. Culture is different, since it's not an expense, and logically it only makes things get better; it's a nonsense to think differently, stating that "I've TOO MUCH culture and now I must have a step back". As with units, I MUST have a chance to keep the current situation unchanged. If I have enough points for another policy, I can choose to pick another branch losing the current progresses in another, but I should also be able to choose "I do nothing", actually spending culture points for nothing.