I don't see the problem really, personally I feel you have to be pretty up yourself if you're unwilling to get over your ego and play as part of the team. If someone's written a song and they have a part in mind, play it. What's the harm? If you feel you can enhance it then go for it, but I guess ultimately it comes down to how respectful the bandleader/songwriter is. If they're a douche to you for playing your own parts then that's not good, but I've never understood the inherent egotism and selfishness of the "I'll only play what I'm asked to if I'm getting paid" attitude.
I understand your position and I don't disagree with you. There's a lot to potentially read into this situation that I, for one, can't know from what's written here.
But, it's fair to say that there are repertory musicians (play what is specified) and improvisers. Some people are one or the other, some people are both. If you've got a "jammer" and want him to play as written (or dictated) then it's probably not going to work for very long. If you ask a typical classical musician to improvise over a set of changes, their head will explode. I don't know if that's what's happening here or not, but all is fine until you have one type of musician, but want them to be the other.
The other thing that I wonder from the description and tried to touch on in my reply, is whether the BL is able to communicate what they want effectively. I too, always give deference to the song writer if they happen to be in the band, as long as they can communicate what they want.