You know, I have wondered that myself. The Citi button is a little different than those preapproval sites, because it's only for existing accountholders, so when you click it too often, they have more reason to be nervous.
As far as the BofA preapproval site in particular, though, unless I'm missing something, the only way to access offers they've given you previously is to fill out the form again. If you fill it out once and receive some offers, I'm not sure how they'd expect you to return to those offers without filling it out again and going through the same process. If you got offers before, you'll have those same offers until they expire, so they obviously do at least keep a record of whether you've been there before.
Good question, though, about whether they hold it against you if you fill out the form too often. I don't know the answer to that one.