Essentially they are trying to lock people in. Mainly they will make a case that it will be cheaper for people to sign the contract and limit your data than to pay the $50 a month. It was easy for me to figure out it was cheaper to buy an iPhone 6S outright in order to keep my unlimited $30 a month plan, then it would be for me to buy an appropriate contract that limits my data.
With how fast internet is getting, it is crazy that they're saying that their data is worth MORE. It should be going in the opposite direction. $20 increase on top of a $30 plan is quite the price increase.