This certainly wasn't the case when I was flying ORD-LAX the other day. As I was placing my backpack under 10D, the FA told me I could't and must place it overheard. I commented that I thought that has finally all been worked out. She said please wait for her safety briefing and she would explain.
Her exit row briefing turned into a diatribe of great length and drama. I've sat in a lot of exit rows and this one was over the top. In a nutshell, it was her job to keep that area safe, and part of this was disallowing anything under those seats, company policy be dammed.
Interestingly, she knew the pax in 12F as a regular, and allowed him to keep things behind her jumpseat under 10F.
In a normal situation, I'd challenge her with logic and known company policy, but we know not to do that on an airplane.
