Well, I doubt that there are enforced rules, but I would personally never use someone's photo for commercial purposes myself unless I asked first. It is an ethics thing to me.
The way I look at it, by posting an image here, I am making
viewing of it public, I am not giving my work to anyone for their own use. At least that is the way I look at it.
There are sites that publish images specifically with permissions for use, but that is a different matter IMO to posting stuff on a forum for others' viewing or discussion. Unless of course the permission to use imagery is a forum rule that is communicated up front before people decide what to post.
I do not think it unreasonable that commercial entities ask first. Most of us here including me would likely be glad to help a vendor in this business if we could, but asking first is an appropriate courtesy.
But OF don't make da rules eh.