You send them your own Raspberry PI, at your expense, for many probably over seas/out of country. You pay them nothing for their service. They're Yet Another Hosting Company.
Wait until you overuse some resources, they're late paying their data center bill (not sure how IP allocation work over there, but RIPE has their IP space, not them), you Pi for some reason goes offline, or the company goes under.
There is no free lunch. Just wait, it'll be proven again.
Sure, you pay to get it back, but they even mention "probably does not cover our costs" for some.
Going under isn't the only concern. A free service which most certainly costs them (bandwidth, power, rackspace, IPs). Even if they're small devices, they add up. And an offering that is losing money will get bumped for an offering that is earning money.
I'm mainly just predicting the future HN post. Panda tears will flow. Just think back to Joyent cutting out the old TextDrive lifetime hosting. People more than likely got their money's worth out of it, but still a large amount of drama.