They force you to come up with a new password that you probably haven't used before and so you will probably forget it.
There are websites that I don't use often where I literally have to reset the password (and go through all the i-forgot-my-password steps) every time I want to log in because they forced me to come up with an overly creative password.
I think most people have two or three passwords for all their apps/services; one very secure one, one medium security one and one low security one (where you literally don't care if you get hacked). It's not the company's business to tell you which of those classes of passwords it deserves for its website.