The reason religious studies are "singled out" is because they are different. If the Catholic Church required priests to have bachelors' degrees in Biological Sciences, there would be no reason to withhold government money for those students.
Different" A degree in social work is different from a degree in mathematics is different from. . .well, you get the idea. I don't see that as a convincing argument as to why one particular type of degree is being 'singled out' and denied aid.