Dunno about US. I am in the UK on some ancient (15+ years, no longer offered) pre-paid plan with 5 free texts per day but outrageously expensive data (something like £1 for every 3 MB). It suits my usage pattern and I top up by £5 only once in 2 to 3 months but... I once blew my entire remaining credit (whole £15, what was left from a £20 voucher I got for Christmas) just by running apt-get update.

Regarding why the phone type matters... It is one of those "unlimited until you use it" things.

I also heard that you pay for receiving calls in the US. Is that the case or is it a thing of the past?