On first glance, it seems like a weird thing to be angry about. Customers get to catch more pokémon, T-Mobile gets a publicity boost, everyone's happy - right? Not quite.
The carrier's offer arguably violates a principle of the open internet that has caused huge debate in the past: Net neutrality. Net neutrality dictates that carriers and ISPs should treat all data passing through their networks equally: You can't prioritise one kind (video, say) over another (torrent files, perhaps).
The most obvious net neutrality violation is a kind of shakedown: ISPs might charge companies and websites for a good speed, otherwise their data will get throttled. If YouTube didn't pay up, then it might find its videos loading slowly for users.
In the US, the Federal Communications Commission has rules in place mandating net neutrality. It isn't popular with ISPs and carriers (as you might expect), but its advocates argue that it promotes competition and ensures the internet remains open and fair.
But what T-Mobile is doing with "Pokémon Go" violates the spirit of net neutrality (albeit perhaps not the letter of the law) in a more subtle way.
No-one is being disadvantaged by the offer - in the short term, at least - and no-one is being forced to pay anyone for preferential treatment of their data. (This practice of waiving data charges is known as "zero-rating," by the way.)
However, in the longer term these kinds of offers risks damaging compeition, which hurts ordinary people.
Imagine a carrier's customer has a choice between two games they want to play on their smartphone. One is a new game from an unknown developer, but they will need to pay for the data they use playing it - and the other is made by a huge incumbent that has struck a deal with the carrier to have their users' data charges waived.
All other things being equal, the customer is going to lean towards the incumbent's game. This shuts out the little guy who can't afford to make deals with carriers to get their charges waived.
(Niantic, the developers of "Pokémon Go," haven't paid T-Mobile for this offer - but the carrier's promotion is a step in that direction.)
Or to use another example: If you have a choice between Netflix and a new streaming service, and the new one looks cool but Netflix isn't going to eat up all your data because of a deal it has with the ISP, which one do you pick? Netflix.
And just like that, the argument goes, competition gets shut down, it gets harder for new startups to enter the market, and consumers suffer from a lack of choice as a result.
Net neutrality advocates are taking to Twitter to criticise T-Mobile's promotion. "Imagine your internet company charged you more to watch indie films than blockbusters," Dieter Bohn, executive editor at The Verge wrote. "T-Mo is doing that for games."
User Amadi said: "T-Mobile is just going full throttle with their two middle fingers up approach to the concept of net neutrality."
And over at Wired, Brian Barrett pointed out the deal isn't even that good - because "Pokémon Go" doesn't use much data. "To fill up a 2GB plan, you'd have spend more time playing Pokémon Go than you would working a full-time job," he writes.
In short: "Pokémon Go" is great, but it's not worth sacrificing net neutrality for.