It's completely unnecessary 99.99% of the time. So if they are forcing the issue when it's unnecessary, I think it's fair to question why. At best they are being really rude and inconsiderate.
Sure, perhaps the dad is being rude and inconsiderate.
I responded to you initially because you said you as a husband/father would be pissed. I found that reaction to be extreme, so I commented on it.
Now you are focusing on whether it is fair to question why and observing that it is rude of the dad. That is a measured and reasonable response, which is totally different from saying you would be pissed(which is a reactionary emotional response based probably on unjustified fear).
A grown man should not make a 13 year old girl uncomfortable in a women's restroom. This is really not a hard question. In 99.99% of situations, you can either get a girl to a stall in the men's restroom without her seeing any penises, or you can wait until a women's restroom is unoccupied.
First off, a typical 13yo girl is uncomfortable about so damn much in their daily life that its unreasonable for everyone to cater to the insanity that is swirling around in their developing brain.
...joking, kinda.
Seriously though, your comment is reasonable. This is the opposite of what I initially responded to, which was that you would be pissed as a husband/father.
At the same time, one quick conversation with the teen's parent would resolve things. Her parent could just say 'yeah, he was there helping his young daughter' and move on. It isnt sexual and it isnt dangerous, so there is no actual justified uncomfort...its perception, which can easily change with explanation and context.