They USED to allow smartphones on prepaid back in the day but then a bunch of greedy , selfish data hogs started doing things like tethering then bragging about it. . Big V got mad and kicked off all smartphones. This has happened on Tmobile prepaid and Att prepaid as well. Unlimited data still exists but for regular phones only, not smartphones. Some "dumbphones" are pretty close to being nearly a smartphone but I think the lack of ability to put even a touchscreen regular phone really kills the appeal of unlimited data . Not having full html doesn't help and most of the prepaid new phones are older, slower 1xrtt anyway.
If they ever come out with their supposed Blackberry plan that was rumored back in January, then they would allow Smartphones. I don't know if it will happen anytime soon but if Straight Talk is supposed to be getting a Blackberry, I supposed anything may be possible for Verizon prepaid.