And who gave this sort of ranger the ability to become a werebear ?
Well, if they are natural lycanthropes, that would have been their mother. (Unless one assumes Bhaal was somehow a lycanthrope while fathering them.
)
If they are infected lycanthropes, it would have been the lycanthrope who bit them first...
The explanation in D&D always revolves around the gods or magic, why ? Because there's nothing natural about it, which is why the stories are
actually in the D&D books... still you should see that the god in the stories are actually the creators of the story or a representation of them as they are the ones controlling every action ever taken, within the rules of the storytelling.
Certainly, as for the first 'source' of lycanthropy, the gods are the creators. Thought that is no difference for most of the races of Faerûn, including humans.
As for the power of the gods and their influence, I think you overestimate them. I said it before, I'm in no way a D&D expert and this is solely my own interpretation of the D&D rules, but metaphysically (outer planes wise) the gods are nothing more than incorporations of different aspects of the human life and mind. Depending on which edition you look at, they are just very powerful beings with high stats, and even if they were more powerful, all of them have counterparts, which, to a certain degree, 'neutralize' each other.
Welcome to the real reality, you need to make the player stay on the good path for them to gain the bonuses from your kit... 
This is probably just a matter of one's point of view, but I just don't want to 'make' them stay on the good path.
As I see it, a ranger can change its alignment just like everyone else can ( actually, there are evil rangers in D&D.. ) and there is no good (D&D) explanation for them to be 'cured' of lycanthropy because of that. 
(On a related note: I believe, that according to the actual D&D rules, a lycanthrope who willingly changes into his/her wereform would automatically assume that forms natural alignment, e.g. a werewolf becomes chaotic evil and a werebear becomes lawful/chaotic good, once they willingly change to their forms.
Thought since the vanilla shapeshifter kit ignores that rule, because apparently the druid has gained so much of control over her wereform, I see no reason why the werebear ranger wouldn't be able to do the same...)