I agree that a boycott is different, and for the NYT the terms are pretty distinct. But when employees are picketing a retail business, the two are much more entangled, so I think the common use of the phrase has shifted away from the precision you're expecting it to have.

It’s not a shift in usage, it’s a term of art. Informally it could mean just crossing the line to enter the business, but unless you live in a turn-of-the-century company town, no one in the union is expecting anything related to that by default.

In labor, “respecting the picket line” is a moral action for union members (or scabs) which by definition couldn’t apply to a spontaneous self-directed consumer boycott.

Not to put to fine a point on it: if you show up to someone else’s labor action claiming solidarity, and then independently decide to pivot the action to a totally different set of economic incentives, you are — almost literally! — a scab.

A scab is someone who works in violation of a strike. It's not a generic term for anybody disfavored by labor activists.

Ouch! Fair cop since we’re nitpicking, but I was trying to be cute about that — the joke started out more like “you’re technically crossing the line to scab as a strikebreaker”. I’m aware that it still doesn’t make sense unless there’s a sympathy action by the strikebreakers’ union, unfunny and unhelpful in the first place, thanks for the correction :)

Edit: Ah shoot not again. What I meant to say was “scabbing as an agent provocateur”. Sorry, I’ll quit while I’m ahead!

It's not even a shift, it's the original meaning. There are/were physical picket lines that you would literally cross to enter a business.

And customers weren't crossing them because nobody buys a car by walking to the Ford factory.

...To our detriment.

In many cases (though not all!), when workers are striking against a retail business, they want the customers to keep coming. Showing the strain that gets put on the system in such cases can be part of the leverage the union exerts.

This is why we, on the outside, need to listen to what the union is asking of us, and not just loudly announce that we are boycotting in solidarity, or "refusing to cross the picket line", if that's the opposite of what they want.

Or don't! It's fine to be pro-labor but for your interests not to intersect with every strike. One might have a different opinion about the moral weight of an NYT strike versus that of the Marriott hotel housekeeping staff, for instance. You might personally find yourself morally aligned with every strike, and in that case you should pay attention to what the strikers are asking. Or you might not. Things are complicated!

Either way: it is not in fact a given that customers are obliged by solidarity to boycott businesses dealing with strikes.

No, it isn't. You can see why people online would want it to be! But boycotts and strikes are different things.

Surely playing a crossword game is only "crossing the picket line" if the workers on strike have asked you not to play that crossword / called for a more general boycott?

I think there's a deeper and more important subtlety here: there's a sort of moral obligation not to break a strike, but except in some specific circumstances, there really isn't an obligation to support a boycott, any more than there's an obligation to put a pro-labor bumper sticker on your car. Breaking a streak and ignoring a boycott are not equally weighted.

(My kid brother is a labor person, so really I'm just venting some stuff here to keep it from coming up at Thanksgiving).

He's in a union and pays attention to this stuff. That's all it meant.

Probably means someone who thinks there's a moral obligation to follow a union-requested boycott.