The fuel coming out of the fuel pump is "not really under pressure?" What's the purpose of the fuel pump then?
I'm referring to that particular hose not being exposed to much pressure. It's the fuel coming out of the return line, not coming out of the fuel pump.
Think of a garden hose. Your home's water pressure is probably, what, ~70psi? But when you hold the end of a garden hose, that piece of hose is not being exposed to 70psi, because the water freely exits the hose. If you put a nozzle on the end of the hose, then yes, the part of the hose
before the nozzle gets something close to 70psi. But again, if there were more hose
after the nozzle, that segment of hose wouldn't be under much pressure to speak of, as the pressure is relieved by the water exiting at the end.
Or think of blowing through a straw. You can develop plenty of air pressure in your lungs to blow hard, but the walls of the straw are not under that much pressure.
So the pressure regulator is like the nozzle. It constrains the flow of fuel from the fuel pump through the fuel rail to ~36psi. But after the pressure regulator, the fuel flows freely back into the tank.
It's not like I'm just making this up ... if that hose were exposed to full fuel-injection pressures, BMW wouldn't have specified a non-reinforced vent hose for that location. 6mm submersible FI hose is available, but instead BMW wants you to use the dinky little tube.