In fact, I think setting appropriate personal boundaries is extremely important. A lot of devs recognize 'hour creep'. You know, stay in a few days until 6, and before too long, it becomes expected. I am saying this as a founder and someone who puts in as many possible hours.
At first, I thought, "Are you guys not really in the game?". But now I recognize that there are only a few people who I should really expect to be on that level, and it should be made explicitly clear from the get-go.
In this case, there is a clear metric that the employee is facing. However, this person certainly adds quite a bit of value.
I think this is worth confronting management over. But I also agree, that if there is a rare occurrence for weekend work, then that is also part of the team. Rather than quit outright, like many suggest, I would confront management very clearly about your personal boundaries and ask them explicitly what is the gulf between their expectations and what you are giving.