> It's surprisingly common for PhD programs to restrict outside activities, although that's probably field dependent.

It's also supervisor-dependent! After I was formally accepted into a post-grad program, I was talking to my supervisor, my then-recent engagement slipped into the conversation and h - a senior bachelor and resident grump - did not approve of me getting married. He thought it would be distraction; I very much doubt that it was departmental policy, but my future research hinged on his whims.