Some docs are just really uncomfortable with DNRs and stuff. They feel like they are failing as doctors if they don't employ every extreme measure out there. This can be a real issue if the hospital is affiliated with a religious organization that doesn't believe in not taking extreme measures.
We had problems with the hospital that husband died at, and that despite his not only having and advanced directive in place, but also having made his wishes clear in his own voice a few days before he died.
I still had to insist, along with his personal doctors, that he not be placed back on the vent when he began to fail. Remember that you can overrule the docs. They work for you, not the other way around.
And, I agree with you: this is just dragging out the inevitable and it is unfair to everyone involved, mother in law included.