Thoroughbreds are not allowed to have ET, it has to be live cover.
I have helped with ET at the clinic I work at, quite interesting to see how the egg is flushed out of the mare and put in with another one. It is done with show horses, alot. I think that beautiful reining mare that just died has other foals because of ET's. I imagine her owner is very very happy that she has that mare's bloodlines to carry one with. I have seen nurse mares used and usually , at least the ones I have been involved with are just given the second foal and gets to keep her foal.
Internet and the humaniacs who use it spend their days trying to make a simple thing bad. Nurse mares are needed, and I am sure some foals are not treated well, but I bet the majority are treated just fine and raised by hand.