Why would Tailscale seek to limit access to their clients, other than where required by law?

The Android client, at least is FOSS. It's hardly Tailscale's fault that people buy iOS devices.

I don't care why. They do nothing to circumvent this so they are not a reliable solution for those who have network participants using the restricted platforms.

There could be a million reasons, but not a technical one — "headscale client", for example, could exist in current hostile app stores, but there isn't one.