Yes, you are correct, you are applying a cantilever load to the vice jaw and it will rock open at the top, vice jaws are often tapered to accommodate this. It happens on all vices, metal ones less, it it still happens.
Designs like the old English ones with the l-shaped bracket and a screw passing through it removes this problem… and creates another that there’s a great big l-shaped bracket sticking out.
Like any tool it depends on what you are using or for as to how appropriate it is, those vice mechanisms work fine for many and have done for a few hundred years I think