Someone who only sends email occasionally ought to buy the low-bandwidth option and save some money. If they choose to buy the high-bandwidth option and not use it to its capacity, well, it's they money to give away.
Certainly, Verizon is not entitled to sell X bytes-per-minute of bandwidth and then decide that you only get x/10 if you surf to a site that hasn't cut a special deal with them... or x/infinity if you surf to a site on their blacklist (e.g. a site that keeps harping on all that inconvenient "Constitution" stuff like this one).
You are right. Verizon reminds me of an “All-You-Can-Eat” buffet that reserves the right to kick you out if you eat too much.