It would be nice if they provided some sort of temperature rating.
Trouble is - these aren't sleeping bags. They're emergency items where you put on all your clothing and creep into your own micro-climate away from the wind, rain and snow.
They're not insulated, and a sleeping bag type temperature rating will be very un-impressive, to say the least.
Bivy bags and sleeping bags are totally different products - the only similarity is that you can (try to) sleep in both.