If there is a KINK never do it for free. I think it's just a Kink some people has. My speculation, correct me if Im wrong.
A) When male climax their anus, prostate and pelvic contracts. I guess when the TS cums when the guys still has the dick in their ass it gives a nice sensation ? Especially when cumming together ?
B) Cum Kink ? Enjoying the visual look of TS cumming and seeing their cum.
I've been with two TS so far and I would not like to see they cum. It would throw me off. LOL