The signs are primarily there for tort liability reasons.
If you have a legal CCW permit, who's going to know you're carrying as long as your gun stays concealed.
Well, there are several responses.
First, why is it permissible for a person running a store that's open to the public to sacrifice MY rights for HIS supposed tort liability? Wouldn't a better sign be: "If you enter this store legally armed, you are agreeing that I will be 'held harmless' at your expense for any and all damages and costs that may result."
Next, you seem to be implying that carrying concealed into a store that has signs against it is a big nothing. I know that people vary on this question, but if I have seen the sign and go in armed any way, I am showing the owner no respect, and I am being dishonest.
Then, you seem to imply there are no penalties involved, "as long as your gun stays concealed." But the ONLY point of having the gun in the store is that you MIGHT have to expose it to save your life. You may then be in for a lot of things: a civil suit that against you by the person you shot or drew down on, with them using your dishonesty in carrying "where prohibited" as a impeachment of character; a civil suit FROM THE OWNER for any legal costs, loss of business, damages/clean-up, mental anguish, etc.
And if you live in a may-issue state, permanent loss of CCW. Hey, maybe a prosecutor thinks he can, like the civil attorney, use your decision to carry where prohibited against you, and goes for a criminal charge--with everything (like loss of employment) that entails.
So, hey, if you
know your gun is absolutely "going to stay concealed", then why bring it into the store; but if you do need it, then it won't be staying concealed--and then, depending on the circumstances, carrying where it was posted not to, could make your post-shooting legal situation much more interesting than you'd like.
The solution: get rid of those signs (ask the owner), or shop elsewhere and carry.