Some Doms believe that one cannot keep your wits about you while Domming AND orgasm, and so they purposefully don't cum.
Also, a man's energy levels dip greatly after orgasm and, it is difficult to keep the energy levels up AND give proper aftercare... Especially when playing with pain or having pushed limits.
Also, typing and tugging could be challenging...
Despite never having played with a Domme, I have a sense (it might simply be a perception) that Domme's would cum without qualms.