And even if in the US porn isn't forced into the .xxx tld by law, for other countries they may jump on it much faster.
Then of course there is the issue of a slippery slope. What other content could be forced onto certain tlds by law?
Sure US legislators can make it mandatory for porn sites to have .xxx, however there are roughly 265 countries in the world. You'll never get every one of them to agree on regulations.
(Although, I agree -- it's a legislative nightmare.)
My favorite is .um, for U.S. minor outlying islands, population 300. It was, um, discontinued.
Agree though that the ever increasing number of TLD suffixes is a bit of a joke. (I think it's a scam to get more money from people, mostly, at this point.)
.xxx conforms nicely to most users expectations of a three letter TLD.