Part 121 Regulations Question

I'm a PPL-ASEL-IA and familiar with Part 91 of course. I'm also a
frequent flier (a 1K in fact....100k plus miles per year). I often hear
flight attendants (FA's), TSA, etc. saying "oh you can't do that. it's
against the security regulations since 9/11." Many times I think or
know that there is no such regulation and all they want is the passenger
to do something without arguing.

Yesterday the middle seat was empty so I strapped my laptop bag into the
seat using the seat belts secured around both shoulder loops, 2 hand
handles and 2 other straps. I've done this many times before without a
problem. For t/o the FA's were fine. For landing, the FA said "you
can't do that." First off, I started to put it under the seat so not to
interfere with a crew member but I also made the comment that it was
allowed. She said, "oh, regulations." I said, "Really? As far as I
know, the federal aviation regulations Part 91.523 and 525 say that this
allowed." Yes, other passengers looked at me like I was nuts for
knowing such stuff. I said, "What regulation is that as I've never heard
of it before." All she said, "oh this is nothing new" but of course was
not able to cite any regulation. I'm not expecting them to know the
details like pilots do but they should know something and should be
consistent. Now I am admittedly not familiar with Part 121 (and the UA
Op Specs, etc.) but as far as I could find, what I did was perfectly
allowable under the FAR's. On the ground while I was "taxi'ing" to the
baggage claim, I checked with a pilot afterwards and he also believed it
was allowed. Was I wrong?

I'm not out to get the FA at all but I've heard such nonsense so many
times that it is starting annoy me when they are worried about a
completely secured laptop bag meanwhile they completely ignore that half
the cabin is on their cell phone "honey, we just arrived" while still
150' AGL on a Cat IIIc approach. If this were a 40 pound child sitting
on my lap and not secured, then most likely there would be no problem
(but that is another story). Further, I realize 100% consistency is not
realistic but using "it's against regulations" and "for security" as an
excuse really dilutes the purpose of the real regulations.

Gerald sylvester