Sprint has been struggling to keep subscriber numbers up over the last few years, and the collapse of the rumored T-Mobile merger didn't do anything to help matters. The carrier has tried all manner of plan incentives to attract customers (Framily, anyone?), but now it's fiddling with the definition of "unlimited." Sprint's new unlimited plan is only $20 per month... as long as you consider 2G data unlimited.
The new plan has 1GB of LTE data, which is the only data anyone cares about. After that, you're throttled to 2G speeds for the remainder of the billing cycle. 2G on Sprint is probably fast enough for email and not much else. Sprint even made a handly little chart to illustrate that its plan is cheaper compared to other plans with a low soft-cap. And yes, it's $10 cheaper than T-Mobile's comparable 1GB plan.
The essence of the deal isn't much different than T-Mobile, which also doesn't have overages. When you go over your cap, you're throttled down to (roughly) the same speed. However, T-Mobile doesn't call this plan unlimited. It's shady for Sprint to say this is an unlimited plan when it's clear that's not what people expect when they hear "unlimited." Sprint is also adding a $70 unlimited plan that's more in-line with the proper English language definition of the word.