Enough people have either implied or stated this directly that its not worth syphoning through everyone's "facts" to determine who said it in this exact way. Either the flags are getting you killed and it is a problem, or they are not getting you killed and you are complaining about the possiblity of a problem. Both are justifiable reasons to complain but "SL trial spawn campers" is not realistic...whoever says it
If I was just engaged in PVP, I would generally want to wait till my flag ran out before I tried to leave the area. Also with friendly contagious flags now I am actually courteous enough, and patient enough, to wait a few minutes for it to run out.
You just broke apart a total statement to argue individual points. Those generally need to occur in the same instance for a player to bring a flag to inferno trial area where teammates are. In otherwords, you pretty much need a 220 with a shadowlands recall beacon active and an inferno garden key, has recieved a team invite close to the same time he obtains his PVP flag, does not wait out the flag because he is a jerk, and does not warn the team because he is still a jerk. There are exceptions (1 out of 1000 pre 220's might have both an inf garden key and the desire to PVP around 220s), but in the end for what you did describe as a common occurance of people being flagged in your teams is not something that I can believe is happening all the time.
My point, I think people are making a much bigger deal out of this than it really is. Flags should rarely last long enough to make it to SL and even if they do dieing there is much more unlikely.