I'd actually saying picking low/high paragon is more unfair than using random.org, I randomly go between 1st/2nd/3rd invited - its always random who gets picked like that as its a speed/system choice and not mine and its not just making someone pay because they happen to be the highest/lowest level.
Given the high number of users, the high number of games, and the quick turnover, using highest/lowest results in a fair distribution. It's also both faster easier to discern than "last in" or "first in" or whatnot, as people can just look over at the numbers and be done with it rather than scroll through the chat log to see if there are any "player_name entered the game" messages after they join. More importantly, it's already well established and accepted by the community, which means no complaints. That's why it's the most common call out during game advertisements.
Any transparent method is good, though, including "last in", and better than adding a "hidden" layer such as going to random.org. I'm not saying any individual here is untrustworthy, but making the rule idiot-proof, transparent, and as fast as possible will defuse any disagreements and keep things running smoothly. Put simply, that's what everyone wants.