Wouldn't it also make entrapment much more expensive if criminals request bitcions to be put into escrow transactions before they make the exchange or even bring out the goods. If the suspects are somehow forced to hand over the private keys they will be incriminating themselves.
How could this be solved by U.S. law?
Silk Road operated an escrow system (they're now looking at multi-sig transactions to replace escrow because they keep losing escrow funds). It didn't hinder the prosecution of SR dealers in any way. Escrow just adds another step and another layer of evidence.
For the most part "is this a possible loophole" type discussions tend to focus on things which either aren't loopholes at all or things which are already in the process of being shut down. If you find an actual loophole and it gets discussed publicly, it's going to get closed relatively quickly because the authorities can read too.