Just to be clear
The risk from allowing law-abiding citizens from carrying in your store is the very small chance of an accidental shooting.
The risk from not allowing law-abiding citizens from carrying in your store is the very small chance they could stop a shooting by someone who is up to no good.
I actually don't know the likelihood of either of these cases. I think for certain businesses asking law-abiding citizens to leave them at home is reasonable, but for others it isn't.
606 accidental shooting related deaths in the US in 2010. I can't find injuries right away, so I only have the death numbers. That also doesn't include idiots who "get mad and blast fools" texting at the movies.
Alternatively, in 2010 there were an estimated 1,246,248 violent crimes and an estimated 9,082,887 property crimes.
Which is more likely to happen, you think?