Sprint would like to redefine the word "unlimited" for you. See, in Sprint's eyes, if you buy one of its new low-cost "unlimited" data plans for only $20 per month, you get unfettered LTE data access for the first 1GB. After that, you'll be forcefully slowed to a 2G connection for the remainder of your billing cycle. That's 2G as in second generation or, you know, the mobile data speed that first launched in Finland 24 years ago. In other words, you'll be stuck using the speed tier that is in the process of being phased out worldwide so telecom companies can repurpose those portions of the spectrum for technology people actually use.
This is more a marketing problem than anything else. Sprint tries to compare its plan to those offered by AT&T, T-Mobile, and Verizon, none of which use the word unlimited. Instead, those companies dress up low-cost data offerings with phrases like mobile value share and simple choice. Even obfuscated by such marketed speak, that language is more transparent than Sprint's. No one in the US buying Sprint service could justifiably subsist on 2G speeds, which tends to clock in at 100 to 300Kbps and takes around 8 minutes to send a 5MB file.
So advertising unlimited access to such a speed tier is nonsensical, whereas the benefit Sprint is actually offering is a way to use a low-data plan without fear of overage charges like one would see with AT&T and Verizon. T-Mobile's $50 Simple Choice plan is almost identical, however it only claims the plan offers "unlimited talk, text, and data" on its website under a banner advertising the 1GB of LTE speeds — not in the name of the plan itself.
Sprint is offering addition LTE data for $15 per GB. Yet at a roughly $40 minimum starting price to use the plan with unlimited talk and text, you're looking at a very small price window before it makes more sense to chalk up $70 for Sprint's new truly unlimited plan if you're looking for more than one or two gigabytes. It is $10 cheaper per month than T-Mobile's similar unlimited plan.
The real danger, however, is that Sprint thinks it can get away describing a 1GB LTE data plan the way telecom companies hawked truly unlimited plans back in 2008. This is anything but.
Update at 4PM, Thursday, October 29: Clarified that T-Mobile advertises "unlimited" data for its 1GB Simple Choice plan on its website, but not in the name of the plan itself.