It may be an insurance issue. Sometimes liability insurance comes with requirements, and if you're a small business you have no negotiating power. That may be why some put up signs but don't attempt to enforce them.
+1
I know insurance co's and liability issues can be a royal pain in the --- for any business and I imagine it's only worse for gun shops. When a GS posts such signs, then I'd also venture to say it's not because they want to but because their ins co. says so.
I just had our insurance inspector come through my shop and you are right, you have no power. They tell you what to do and you do it.