I think at some point they DID restrict this. Then they relaxed on it. I think it's sensible to allow at least some simultaneous viewing. Sometimes my wife watches something on her phone while I watch something on the TV or vice-versa. It probably happens maybe an hour or two worth of content per week. I think that's reasonable to not restrict

However, I don't think it's be reasonable for the people that are sharing one account so that 2+ people are simultaneously viewing content from different locations more than 10 hours per week