I'm actually the opposite: I tend to avoid being paid to speak (one time I was actually paid, but I didn't cash the check), but if you want to record a video of me speaking for your usage on your website, then I consider that something you really need to be saying up front and which you should probably be paying for (and thereby which I often just won't do, as I usually avoid being paid; my standpoint on avoiding being paid will probably change over time, though).

The first issue is that it isn't really "publicity" for me (to respond to your linked point #3 about wanting publicity), it is free and valuable content for the event: to the extent to which I obtain publicity, I get it from the people at the event that I am addressing with my talk, not from the people who run the event; the second one is that recording the video to go with the slides doesn't mean that the content will now be understandable, as I feel truly awesome content is often somewhat interactive (I try to break the ice quickly to allow questions to interrupt me); and, finally, it has a severe cost on the talk... when you are speaking with a smaller circle, you can tailor your talk to directly meet their needs (I make it a point to meet as many people as I can before I give a talk to ask why they are attending the conference, what they wanted to get out of it, what company the work for, etc. and I then retarget my talk to directly serve those interests, use language appropriate to the audience, and sometimes can come up with specific and more useful examples), but when you know that thousands of random people are going to be watching this recorded later you suddenly have to talk to a general audience and have to be more guarded about stories and predictions (as you will certainly be called on them years later in drastically different contexts)... it really just ruins the experience.

(edit:) In essence, it is actually a different medium from a "live" talk: a talk that is being recorded is to a different audience (undermining the goal of having a conference tailored to specific topics and interests) and has different constraints (difficult interactivity with either the audience or the slides, a tradeoff on movement on stage vs. framing of the camera), in the same way that a slidedeck without an associated talk is a different medium from a talk; you aren't really giving a talk to a conference anymore: you are giving an address to the world.