I've been both a free speaker, and the person trying to arrange other free presentations. Here's the thing: in both situations, at least in my line of work, the speaker is getting paid, just not by the organization they're presenting to. A technical presentation is a form of marketing, and is usually budgeted as such.
My vendors love to give
Because I'm on the consulting side of things, I'm not usually selling a particular product in my presentations. But if I've worked on a cool project and learned something useful, and I share it in some sort of public forum, I'm educating others and also keep my name out there as a smart person who does interesting stuff. Sometimes I present at conferences, and sometimes a big client will request a presentation for their own technical staff. I'm paid for my time (more or less) by my organization, and I may get a conference discount, but I'm not getting anything more.
Of course, sometimes I do volunteer. Maybe it's for a science career fair at the nearby high school, or they're short a speaker for the local branch of a geological/environmental society meeting. But it's not something that's keeping me from my paying job.