My theory:
I once had a RBTA "eat" a fish. I thought the same thing as you, but now have a different theory. BTA's don't have that potent of a sting (relative to other nems). Normally, healthy fish should be able to avoid a BTA and even be able to get away if they get a slight sting. However, if the fish is sick or unhealthy, it won't be able to escape a slight sting and get eaten.