Yes. You’ve hit on it. That is why they annoy me. It feels like taking advantage of a captive audience.
Although, just from my analysis of free speech, I shall defend it as an expression of freedom of speech. It’s still free speech, I think. And I’ll defend the right of people to write whatever slogans they like on whatever property they have.
I’ll be annoyed about the delivery method. I shall be annoyed about that. I shall wish they made arguments that better fostered communication. But I shall disagree about whether bumper stickers and similar are expressions of freedom of speech, because I think they must be, even if they’re annoying and restrictive.