If you watch their videos, they explain it. They don't want to call it a free product because they're working hard on it and should be paid for it. But it is a perpetual trial license for those that don't have the means or ability to pay.
How is it suspect? They call it source first or some bullshit but the point of the liscence is to stop people from putting fake futo apps with ads on the play store.
But the code is still completely open, you can look at it as much as you want
OP asked for FOSS, and if this keyboard had met OPs other criteria, it would have failed the FOSS check (it's a source-available license). It's also a roll-your-own license and a very very short one at that. It's missing a lot of key protections for both the company and the consumer.
I'm pretty steadfast on using GPL software wherever i can, especially for something as mission-critical as a keyboard. Non-gpl projects have a tendency to get bought up and relicensed or corrupted in some other way over time (sometimes a very long time, but time nevertheless). I'll make exceptions for things that are less critical, like games, but core system must be GPL or offer equivalent protections for the end user.
Source-available is still good for auditability though, making it more secure in the short-term.
I just realized, you're an ML user. Aren't you supposed to be hyper-communist? Stallman was probably one of the most communist in terms of software development.
If it's not source-available, then it's somewhere between source-available and OSS - that license is very aggressively in favor of futo and against the general public.
Just to clarify the scale:
Best: FLOSS (GPL, etc)
Better: FOSS (Apache, etc)
Good: OSS (MIT, etc.)
<--Futo is here
Bad: Source-Available <--or here
Worst: Closed-Source/Proprietary