I don't think it's that conspiratorial. I think FOX likes Stossel because he's a good on-air talent. Same with Napolitano.
I think that, yes, they tolerate the Ron Paul and libertarian inclinations of Stossel and Napolitano because they realize it's a bad idea to completely alienate the Ron Paul crowd (they already saw what that looks like in 2008), but I also think they tolerate them because they LIKE both of them and think they are talented on-air personalities. So it's not so cut and dry.
If you read Stossel's latest book, he talks a bit about his experiences with FOX, and contrasts that with his experiences at ABC. It's clear from his writings that FOX, in contrast to ABC, has been very friendly, supportive, professional, and collegiate.
I'm not saying FOX isn't still the devil, but I don't think it's the case that the FOX execs were sitting in a closed-door meeting one day and said "we need some libertarian voices, let's hire Stossel and Napolitano and pretend we like these guys".