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.
Edit: Ah shoot not again. What I meant to say was “scabbing as an agent provocateur”. Sorry, I’ll quit while I’m ahead!
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.
Either way: it is not in fact a given that customers are obliged by solidarity to boycott businesses dealing with strikes.