Is it a standard thing for presendtial candidates to campaign for other candidates

[Igel]

4. They do, from the president on down.

In fact, Obama was slammed a time or two for doing less campaigning for things like senators or governors than most.

It goes to the idea of a party. The (D) party doesn't exist just to serve a particular candidate. The party is a group of people who are loyal to it and its members and work to help it and its members.

Abusing it like we usually abuse a commons will lead to the same consequence as abusing a commons always does.

Misunderstanding how a party works can be difficult for a rugged Independent. Plus, not all parties work like American political parties. In some cultures, the party dictates its platform to members, they're the members' marching orders . Following policies that aren't correct, failing to tow the party's line, is a serious matter that can lead to censure--hence the "politically incorrect" phrase, which just meant "not correct according to party policy" (in Russian, "policy" and "politics" are, or at least were, pretty much the same word). In those cultures, it's important to control what the party line is. In the US, the "party platform" is an expression of the collective values at a given time and place, but far from normative.