Because now a treacherous bastard manages to con you and join your gang, worst he can do is lead a small group into your base. If radio signals could be used inside a base, then the griefing possibilities are endless.
Imagine a 5 people small band getting betrayed by a sixth new member who invites his 40-people gang to just make your life miserable for the hell of it. They could wipe your base clean, keep even a temporary beacon on forever by just spamming /h every 5 minutes, bring 10 supermutants to your base entrance, and basically make your characters useless by denying you to use your terminal to resign from your faction. Right now it's enough that you kill the first group, kick the player, and kill the second group together with him. With a beacon even if you did they'll keep coming and coming. And won't even need the traitor to enter your base.
Bad idea. The game already has a lot of asshats who aren't satisfied with just playing, but want to spoil the game for everyone.