Don't believe it is against the law, it is the property owners posting signs stating no carry allowed. And in some states (not Utah), those signs carry the force of law and if you are caught in the location with a gun, you can be arrested and charged.
In addition, some states that have written into law the enforcement of private enterprises to post no handguns they might also have a justifiable defense law written into the code. A good example is the state of TN. If a business posts a sign, as described by the TN code, and you are found in that place with a handgun, you can be prosecuted under the law. However, if during the time you were in the business, and even if you were there illegally, if you used your weapon in a justifiable defense of yourself or someone else; you cannot be charged.
And I'm not saying I would not shoot, be sure I will, but I am saying it could be very tough and it will be over very quickly.
You guys in CO need to get your scored lowered to ZERO, like UT, AZ and AK. Fifteen is way too high....