T-Mobile may be an “Un-carrier” but it’s not a charity — it’s here to make money. So while its new Data Stash plans that let customers on capped plans roll over unused data to the next month are good deals for some subscribers, they also come with a side benefit for T-Mobile: They give wireless subscribers an incentive to sign up for more expensive monthly plans.

FROM EARLIER: T-Mobile’s bold new ‘Un-carrier’ plan unveiled: Data Stash lets you keep unused data each month

Ars Technica’s Jon Brodkin notices that to qualify for your own Data Stash, you need to subscribe to a Simple Choice Plan with at least 3GB of data per month.

“The offer is not available for T-Mobile’s $50-per-month 1GB Simple Choice plan,” Brodkin writes. “Smartphone users who subscribe to that plan will continue to be throttled after they exceed 1GB in any given month, under T-Mobile’s policy of slowing data speeds down instead of charging overage fees. The 3GB plan costs $60 per month. Data Stash is also not available for T-Mobile’s cheaper Simple Starter plans, which cut users off from data when they hit their monthly caps.”

Now, there’s nothing sinister about this. T-Mobile is making its more expensive plans more appealing in an effort to entice more users to sign up for them. Getting a plan with a Data Stash only costs $10 more per month than a low-end plan that doesn’t offer a Stash. All told, the $60 monthly plan with the Data Stash is offering more value now than the $60 plan that didn’t offer any rollover data.

That said, if T-Mobile can get more people to sign up for plans that qualify for Data Stashes, it will deliver a nice boost to its quarterly average revenue per user (ARPU) figures — and that’s a side benefit for T-Mobile that the “Un-carrier” probably won’t advertise.