I was able to find the law I was thinking of, and it was passed in 1956, so not terribly recent, no. Apparently it's because the BHCA defines banks as entities

I was able to find the law I was thinking of, and it was passed in 1956, so not terribly recent, no. Apparently it's because the BHCA defines banks as entities marketing both checking accounts and commercial loans, and Discover Bank makes no commercial loans and is thus not, under the terms of that law, technically an actual bank. (Also, apparently the BHCA restricts banks from operating across state lines, too.)