The moment one deprives others of their share, one is engaging in PVP, because, in the postulate, one has agreed to the sharing. More correctly, one is being a jerk and a liar, and is genuinely acting against the other players as much as the character is working against the other characters.
I've booted players for that.
Yeah, gonna disagree with this. This seems excessively....hyperbolic to me. When you agree to sharing loot, there's an implication that all party members will also be sharing in participation to get said loot. If the rogue is able to find something where only they were at risk or participating to get the loot, let them decide.
Let's say you're in a committed relationship (married, partner, whatever). Most committed relationships have a social contract of sharing. If you happen to find $10 on the street, you're actively acting against your partner, and are a jerk and a liar if you decide to just pocket it? If my partner finds $10 outside, she's not actively working against me to keep it herself. If I got upset about that,
I'd be the one who is a selfish jerk. Yeah, we agreed to split all the bills and finances, but that was something special she got to unexpectedly enjoy. It would be her decision to share or not.
Actively stealing from PCs is always a no go in my games, but I do not consider finding lost or overlooked loot, or a pick pocket to non PC member PVP because finding extra lost loot somewhere is not an active group thing. They weren't involved at all. Only the rogue. And it's a class feature of the rogue, especially the thief rogue. You're essentially telling the player that they are forced to always use their class features to help the rest of the party and not themselves. Talk about taking away player agency.
Imagine telling the casters that they can't cast any spells that don't actively help the entire party. Imagine all the down time activities eliminated by such an edict? Most of my players have individual backgrounds in the game, and during downtime a lot of things happen that connects to those backgrounds. You're saying that no PC can do anything on their own time that might further their individual story unless it also helps the rest of the party as well, or else that player is a liar and working against the party?
Again, pick pocketing or finding otherwise overlooked look is a class feature of a rogue. Let them have it as long as it's not adversely affecting the rest of the group. I certainly wouldn't use such strong language to call them liars who are actively trying to hurt me.