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In fact, the US government can't.  The US Supreme court defines most pronography as established by community standards.  Child pornography is an example of an exception.  It is difficult to apply community standards to the internet.  The case law can be a problem.  I remember a case where a satilite broadcast was shutdown because it violated the community standards somewhere in Louisiana.  The community standards of the origin and a lot of other communities were not violated but there was no way to limit the broadcast.  A similar problem exists on the internet.  It is a thorny legal issue being persued over online gambling.  A Navada site where gambling is legal is accessed by a cop in New York where it is illegal.  The Nevada site is sued by New York attorney general.  The sites opening page requires you to affirm that gambling is legal were you are.  The cop lied.  The last I heard, the courts said the opening page was not enough. 

But to answer you question, no I don't want the government doing that.  I wish to guarentee your unlimited right to post naked pictures of adults in the .xxx or .sex or .whatever domain without fear of prosecution.  Post them in the .com domain at your own risk.  If you do so in another domain, you may or may not be prosecuted and you may or may not win.  If you are prosecuted and you win, you are still out a ton of money.  I don't think those in the business of pornography would want to take that risk.