It's apparently what happens if you don't give him the book or encourage him not to talk to Mystra at all - though I have never tried this myself since I really don't want this crown just lying around, ready to be snatched by the next tyrant.
The "trouble" with Mystra might also be, that she herself isn't an evil goddess and apparently a fairly popular character. So some people seem to get upset that Gale doesn't grovel enough in front of her, taking his dialogue about it being hard to feel affection for a lover who condemned you to death as "bad mouthing the ex". I kind of like the weird relationship parallels in the story: they either become spiteful exes (Gale takes the crown), just part ways (Gale leaves the Crown in the river) or he accepts that she as an entity/guiding principal is something he holds dear, while she makes an abysmal partner or source of affection (he does the right thing and gives her the Crown and uses his position as her Chosen to do good stuff for the wellbeing of others.) They could have explored that more, that is true and probably things have been removed or toned down too.
