You might have noticed that no one is his own man in 1377—everyone's working for someone, and if they aren't, they're working for God. Enter Bear, who is definitely his own man, though he pays a high price for it. Bear's personal history attests to his attempts to control his own destiny. He left the religious order his father sold him to in favor of joining a troop of mummers (dude literally ran away to join the circus), then fought in France as a soldier. He escaped the Great Mortality by going as far north as he could to the islands off the coast of Scotland.
Now Bear makes his living as a traveling jester and wants to share the personal freedom he's found with everyone else. He says of the king, "'It is as it is,' is his motto. Mine is, 'Let it be as it may be!'" (16.34) The problem is that this is treasonous talk, and even treasonous talk can get a guy executed. But Bear dreams of a better world, and all his education has left him unwilling to believe that things are the way they are and there's no changing them.
Despite Bear's initially gruff exterior and the fact that he forces Crispin to work for him, he's a good guy who becomes Crispin's adoptive father by mutual agreement. Bear is literally the father Crispin never had, and he turns out to be a better all around guy than Crispin's biological father, Lord Furnival.
Let's look at the kidnap situation, shall we? From Crispin's viewpoint, Bear essentially takes him prisoner—but this may not be the way Bear sees it at all. Sure, he says all the right things about making Crispin his servant, then his apprentice, then his son, but Bear knows that the reality of the world they live in is that without a strong male figure in the picture, a child is extremely vulnerable to all sorts of danger. As Bear says himself:
"By the love of Saint Arnulf the King […] you could do much worse than being bound to me." (19.19)
In other words, it seems that Bear works within the existing system—one in which he can bind Crispin to him simply because the boy's a serf and Bear finds him—to keep Crispin safe. Sure, it'd be nice if he fast-forwarded to the whole adopted father bit instead of initially making Crispin his servant, but we're also not sure Crispin would go along with Bear otherwise. It's not like he knows the guy at first, after all, but he does know the rules, and the rules say he has to stick by Bear because Bear says so.
Bear's savvy enough about the ways of the world for us to suspect he works the system instead of truly upholding the very thing he risks his neck to dismantle.