Really? Taking a picture of someone's feet against their will and posting them on a forum where people are going to be fappin to them, without their knowledge, isn't a crime? I have no idea one way or another, but I would be surprised if it wasn't.
Generally speaking*, it's not.
I did some research on this stuff prior to starting my video business. I didn't do it with the intent to be able to cover my ass should I get caught snapping candids, but rather I felt I needed to know what the rules were so that I could follow them, vis-a-vis securing model releases.
There's a lot of discussion about this stuff (in a non-fetish context) on photography websites. Just Google "model consent form" and read your heart out.
In summary, though, no; you don't need a model release to take someone's photo, or even to sell it, at least not in the US. The article I read used shooting a kids' soccer game as an example; a photographer is, legally, in the clear if they don't secure releases from every player/member of the crowd, even if they bundle all of their photos into a book and sell it.
The lone exception, it seems, is photography that implies endorsement. So, for example, if said photographer took a photo of Timmy shooting a goal, and then added a caption that said "Timmy votes Republican, like all soccer players do!", that's a no-no, and Timmy (or his parents) are justified in doing whatever it is people do when they need to seek legal recourse in such matters.
However, fap snaps (as I like to call them) are a whole 'nother kettle o' fish because of a little thing (at least here in the US) called 18USC2257. It's the law that makes sure porn is produced all legal-like and that all participants are over the age of 18. Anyone filming video of a sexually explicit nature (the exact conditions of which are spelled out in the code) MUST keep detailed records of their models in accordance with said code.
Do foot fetish videos count? The jury is out on that one, AFAIK (that's a pun, son) and foot fetish content, strictly speaking, does not fall under the conditions described in the code... but as most of these clowns are posting said videos/photos on fetish sites, where pictures of minors are strictly verboten, and/or selling them on sites like Clips4Sale, an ambitious DA could very well make the case that said videos/pictures are, in fact, porn, and thus need to be 2257 compliant. Oh, and if you're not? Congratulations! You just committed a felony.
So, the thing about candid fap snaps is that I'm pretty sure none of these guys are keeping 2257 records for these random girls on the street. And that's a problem. Not to mention, yes; there's something just inherently stupid-creepy about guys following women around with video cameras so they can masturbate over the footage later. Regardless of the legality, no woman I've ever met thinks otherwise, and that in and of itself is a pretty damn good reason not to do it.
* Laws vary by jurisdiction. I am not a lawyer.
