Because they need to accurately track your logins and trading activity. They just don't want you doing anything that could possibly cheat the framework they have in place.
How does using a VPN "cheat the framework"? You still need to log in with your own credentials. Only your IP will change. But that could also happen if you're traveling. So does that mean you can't trade while traveling?
It’s one of the biggest, yes (it owns Oanda), but it’s not a futures funding firm, as Topstep is. It’s a counterparty (just like a CFD broker is).
I don’t think it does, with Topstep, but there are certainly some other funding companies for whom that’s enough of an issue for them to specify that they require (at least) “advance notice/discussion” if you’re intending to trade while travelling.