I've noticed it is fairly common, enough that I've encountered it multiple times as a player and as a GM, for players to erase their characters from the game as much as they are able before exiting the game. They remove descriptions, delete character sheets (when they have access), and even remove their character portrait.

Sure, there are steps you could take as a GM, making copies of everything, to lessen the impact to the game if you plan on continuing to use the character until you can naturally walk them out of the story (or continue with them as an NPC ally or party nemesis). My question is, in terms of gaming/RPOL etiquette, do you find these actions kosher or are these players just being petty?

I think it depends on the situation, but it could also be considered the same as a GM marking his game as Sole Ownership, meaning that the player is not necessarily being petty, but just doesn't want someone else playing as his/her character