I'm not sure what you're arguing. Is it that IP protection laws are immoral, or that they're impractical? GP was making a moral argument (I think), you seem to be making a practical one.
So assuming that you are making a practical argument: should there be no laws that are hard to enforce? Let's take domestic abuse, or child abuse. Hard to detect, even harder to prosecute because there is usually little physical evidence. So should we just throw our hands in the air and go home?
"Such laws are still absurd in a world where there is no practical way to limit the distribution of such ideas."
Well a largish number of raids over the last years on sites hosting torrents, causing these sites to disappear, would say that you're wrong. Sure, it's hard to completely restrict all illegal distribution. That's not even a practical goal. But with TPB down, it's become quite a bit more difficult to find illegal content for people who are occasional torrenters (i.e. not people who are members of private sites, or who use IRC - a tiny subset of all people).
People have been saying for years 'you can't shut down the torrent sites!'. Well there aren't a whole lot of them left, are there? So it does seem that IP law enforcement isn't as impossible as you claim it to be.