Why does it seem to be okay to bend your wrist in an OHP (many many tutorials show it that way) whereas it's highly adviced (even from the same people doing these tutorials) to get your wrist in a straight line with your forearm when performing a Bench Press?
I think maybe you're mis-interpreting the tutorials or the tutorials are wrong.
The weight of the bar should fall on the base of the palm which places it right above the arm. This is true for both bench and overhead press. That is the goal you are trying to accomplish regardless of what cue you use to make it happen.
On the left of the image, you see that the bar is not resting over the forearms. The wrist is flexed far too back.
On the right of the image, you see the that the bar is directly over the forearms. The back of the wrist is still going to be flexed back slightly though. The front of the wrist, however, is pretty straight compared to the forearms.
I think the confusion comes in when someone is told to keep their wrist straight, they focus on the back of the wrist. They focus on this, but it rolls the bar too far forward. Though, with sufficiently heavy weight, this actually becomes very difficult. In trying to straighten the back of the wrist, you fail a little but end up in the correct position.
There's a couple of factors: