Nothing inherently wrong with that sign. If they have a lot of people that stand the entire game then they should have dedicated sections for these fans. You know, the stands.
I've been to soccer stadia in Europe where I've been in these sections. I've also been to other sections.
For football it's fine to stand for exciting plays & big downs. Other than that, I would expect most to sit down most of the rest of the time. It's the "normal" thing to do. If the 49ers expect people to sit all of the time then they're FOS.