ouch..
but yes, this is just how the all time awd transfer case works. you have a parking paw in the transmission, not the transfer case. so paw lock the output shaft of the transmission in park. but the transfer case isn't locked. once you took the drive shaft off the front, it is basically in neutral.
best way described it is think of your rear diff of a 2wd. put the truck in park, jack up both back tires and they will turn freely but on the opposite direction. same with the transfer case. you didn't notice but once you popped the drive shaft off, it was spinning the opposite way of the one going to the rear end as it rolled over you.
the other kind of case with 2wd, 4h/4lo 4x4 transfer case it different. when it's in 2wd it shouldn't roll away.