Common enough even in tech that I've both had contracts try to demand this, and had contracts explicitly rule it out being presented as evidence of how great the company was.

Seconded. I had to be very careful to work on side projects completely divorced from my main job for a spell, and had to get legal approval first.

The common attitude of companies is that they’re paying for the whole of your life inside and outside of “work”, and these Unions are a response to that encroachment (and associated under-compensation in general).

Good on them. Best of luck negotiating a fair contract!