It's not exactly impossible to go after Doom legally. We've seen it done a few times, like when the Mighty Avengers did it prior to Secret Invasion.
But yeah... that's not necessarily a fight you want to pick unless you're really prepared to deal with the consequences. A war with Doom will make conflict with SHIELD or the Avengers potentially look like a picnic. That's not to say they're not necessarily justified in going after him if that's what in theory they chose to do... but they had better be sure it's worth it.