They are open source too:
https://github.com/tailscale/tailscale
edit: Only the client is open source. See clarification below.
replies(2):
edit: Only the client is open source. See clarification below.
There is an open source alternative called headscale [0]. The main downside is that you'll need to run it.
The closed source centralised control server has other potential issues though, and it ends up being up to the user to decide what's the right balance of security vs convenience.