Raymoland

We get it, you don't want anyone muscling in on your exclusive first class. The reason MSPEconomist tried to educate you is that the situation is different between a deadheading and a jumpseating (commuting pilot). (As was pointed out to you, either could be on the last flight of the night since neither of these things only go TO assignments, they do get to go home as well). A DH will have a confirmed seat and there will be rules around whether or not that DH is eligible for an upgrade. I would agree that if this pilot was on a DH, they should have known better than to try to poach a seat.

However a jumpseating pilot is often not assigned a specific seat as they generally board last. I can't tell you how many times I've been told by the gate agent to "take any open seat" as they are just trying to get the flight closed on time. I also can't tell you how many times I've been given a good seat (a last minute no show), walked down to take it only to find a "rude" passenger has switched into it because they thought we are done boarding. The general protocol in this case is to take whichever seat the other pax has moved out of, of course this is usually a middle seat. Do we complain about that? We do not.

Lastly, jumpseaters must check in with the Captain. This is partly a courtesy but the Captain also checks your paperwork to make sure you are entitled to jumpseat and of course if you are going to ride in the actual jumpseat it is common to ask nicely if that "is OK". You shoot the breeze a little depending on how busy the Captain is, whether you know him or not. A nice Captain will often say something along the lines of "take whatever seat you like" or Talk to "purser" to see what they've got open or any number of variations along those lines. Many times this is just the captain wanting to be friendly - they don't usually know what's open or not but they are just being hospitable. It's perfectly possible in this case the Captain may have said "grab a first class seat, "purser" won't mind.

Now regarding other discussion that came up, the Law couldn't be more clear. 14CFR 91.3 states The pilot in command of an aircraft is directly responsible for, and is the final authority as to, the operation of that aircraft. This is also replicated in Delta's flight operations manual and does get used from time to time by Captains to point out that if a difference of opinion exists between any two parties as to the operation of the aircraft, the Captain's opinion is the one that will win the day. However this is used very judiciously in most cases by most Captains because they don't want a reputation as an ....... and don't want to piss off the cabin crew whom they will be likely to have to work with in most cases.

So in the example of above where the Captain may have said to the jumpseater "grab a first class seat" and then the purser says to the jumpseater "Covid rules mean you can't sit there" the most likely response from the Captain would be "sorry man, I tried". All this is just by way to point out that the pilot in this case may not have been quite the arrogant ... you have attributed to him and quite likely may just have been going along with what had been told or suggested to him by one or more parties along the way. Maybe instead of condemning him on the internet you could have tried a little harder to understand what may have been going on about which you, apparently, know very little.