"Letting it happen" means that it's something that can be controlled. A decentralized protocol like TornadoCash is not administered even by Roman. Yes, he's a developer, and has the most know-how around the code, but it's still decentralized. He couldn't just turn it off, make appropriate changes and turn it back on. As well as that, developing functions that would filter or deter money laundering is not a simple thing to make on-the-fly (as even today, AML scores are subjective and not sure-fire ways to detect money laundering). With this in mind, it might not be as difficult as it seems to defend himself.
America's govt didn't change their mind they're taking him to court because they say he's guilty. If it's easy like you say he'll be freed but if the case's strong against him he's going to have difficult days with the judge & jury. The case against Tornado Cash's dropped so they should drop it against Roman Storm.
While the explanation sounds simple and straight forward, there is always going to be the intention to prosecute/vilify him combined with the added intricacies of the law, which always complicates things and creates challenges. Otherwise, I agree that it should be dropped too.