Yeah, I'm pretty sure the majority of the foals aren't just killed after birth. And what "other jobs" are you referring to in regards to the TB broodmares? Mares are not bred while they're still being raced. There wouldn't be any way to keep them in racing condition while they were pregnant. Yeah, a lot of times they'll re-breed the mare when she comes into her foal heat, but that typically doesn't require the existing foal to be given to a nurse mare. The only time I've heard of a foal going to a nurse mare while it still had a healthy living mother was if the dam rejected the foal completely.
I don't know if you've been around a TB breeding farm, but after they're retired for breeding, the brood mare's only job is to make and raise babies. Where are you getting your information from? Not everything you read online is true.
The TB broodmare is usually bred directly after she gives birth to her foal. Which is considered "her job" to be bred and have tb foals. So once the foal is born they take the broodmare away from the foal to be bred again. This leaves the foal without its mother and without milk. Which would mean a nurse mare would be needed to provide the foal with milk. Which leaves the nurse mare's foal without a mother and without milk...usually left to die....unless someone comes a long and rescues the foal.
Also the Jockey Club deems it to dangerous to put newborn foals in a trailer with the mother so they are seperated IF the broodmare must be trailered somewhere. This also leaves the foal without a mother and in need of milk. Which would mean a nurse mare would be brought in. Which leaves the nurse mare's foal without a mother...and usually left to die....unless the foal is rescued.