You are correct. Typically, if the pilots run into a critical emergency and have the chance, they'll try to pitch the aircraft upwards to give the navs a chance of survival after ejection, but it's pretty rare that's feasible.

I don't know how it is anymore, but it apparently used to be an unspoken rule that if the downstairs crew couldn't eject and survive, the upstairs crew wouldn't either.

As to why the ejection system is like this? Because the B-52 was originally designed as a high-altitude bomber.