It is indeed obnoxious, ridiculous. asinine, and any of a number of other pejorative terms that you might care to employ.
What is the point? Or to quote Basil Fawlty, "What is the bloody point?" I couldn't begin to imagine. I just know that's what the regs require. At a guess, and only a guess, this restriction only applies to custodial accounts because in the dim and distant past, 403(b) were annuities only, and so when this provision was instituted, annuity contracts were "grandfathered."