There is no way of doing this unless you jailbreak (i dont know if jailbreaking allows this option either, but it seems easy enough to do). I honostly don't know why Apple does that, but on the other hand, my friends have regular ringtones for there text messages and it does get really annoying. Plus it makes it harder to tell if your getting a call or text message. I do agree with you and wish there was a way to get self made tones for text messages, but those might be things going through apple's head when they were doing that. Atleast they gave us more options than the original one.