You do. I used it like that for many years, until I could afford the PC version. The raspberry pi version is version-capped though, to whatever they have released up to that point for that platform.
Are there restrictions against running it on a more powerful arm64 server?
This is an interesting question (which I don't know the answer to) because the Raspberry Pi hardware does not itself come with a license for Mathematica or the software in question. Rather, it's part of the Raspbian distribution. I think there's a decent chance that you agree to an EULA after installing the software that would prohibit running it on other hardware, but it would be interesting to hear something definite.
At one time, it would test for the existence of Pi-specific devices, but it’s been a long while since I ran it there.