the nature of pollution is doing that, even once. The nature of harassment is continuing to do something

In the same way one piece of litter once is considered polluting, the idea is that approaching a person once is considered a kind of harassment. It's not what one individual does, it's the net cummulative effect of many.

the right to be able to say anything you'd like trumps the fact that it might bother someone

But you admit there are limits and these limits are arbitrary depending on culture. Shouting insults or harassing a woman were not crimes years ago. You must admit we are discussing an arbitrary line so declaring an absolute right is pointless. All rights have limits.

If a single person is being constantly catcalled, there's probably a reason for it

This is victim blaming. The same could be said of any crime. In some cultures just being an attactive girl alone is enought to get called at, add a male companion and they leave her alone, how unfair is that?

almost every case, these comments are transient, and the person is never seen again

Not so. The habit and culture of catcalling or insinuative greeting is both intimidating and validates a stalker. If you allow "well hello there" you can't stop "nice ass" so you also can't stop "nice ass" from one metre away and now it's a matter of statistics to get her ass grabbed, this is what is happening. The guy that said "well hello there" from across the street contributed to an atmosphere that enabled an ass grabbing. The "it's only words" argument is very bad, there are many restrictions to words (slander, false declarations, etc).

the whole "meat is murder" campaign. I see those posters from time to time. I don't intend to stop eating meat, and the poster doesn't in any way lead to that.

We already discussed this. The objective of a poster like this is to create awareness, not enforce an action. If you saw an official street sign that said "meat forbidden beyond this point" you'd take it differently. The poster just creates thought. Some people on the fence might move one notch and eat less meat, those that already don't eat it might feel invited to explain why and others will ignore it but not be amazed if someone comes across as a vegetarian.
The anti-harassment campaign is lowering tolerance a bit, in a large community, to something that bothers a lot of people. I am not sure you have undergone constant and repetitivd street harassment, I have and I understand why it's best if it's discouraged.

Think that just this discussion is the product of the poster. It works.