[bsimpson]

The difference is that someone in real life can see your ID without recording it.

The internet's history of anonymous-by-default and the taboo that people can still feel viewing pornography are both factors that make ID verification laws very touchy (also reasons free sites like PornHub are popular in the first place). Compound that with the perpetual security breaches that we've all been victims of in recent years, and you can see why these laws have strong opposition.