Exactly, and it clearly states one of the things you cannot do, is carry a handgun on your person, unless you meet one of the specific exceptions. And having permission of the property owner is not one of them. So he can give you permission to carry and you can avoid a PC30.05 trespassing charge, but not the UCW charge unless there is a specific Case Law for it.
Can a property owner give me permission to carry drugs on his property? No. Carry child porn? No. There are an infinite number of examples of things a property owner can not give you permission to do. Carrying a handgun is one of them.
The best way to understand Texas handgun law is this. In 1871 they banned carry of handguns. No exceptions. None. Over time folks got arrested and appealed their convictions. Some won, some lost. The wins became know as judicially carved exceptions. Some became common and were codified in law. Others not.
Here is one of the best reads ever on Texas gun laws:
http://www.stephenhalbrook.com/law_review_articles/texas.PDF
Is a gun illeagal for a person to carry on property they own or that they control? YES. The INVITATION to be on that private property legally gives control and consent from the owner to the person invited.
Permission CAN be granted, and IS granted every day all acorss the state.
Your mention of child porn and drugs is laughable...we are talking about something that a person CAN LEGALLY HAVE on PROPERTY THEY CONTROL, OR HAVE BEEN GIVEN CONTROL OVER.
To say that I, as a property owner CAN NOT GIVE CONTROL over to another for them to enjoy an activity that would be legal on property that they own, control, or have been granted by invitation control over, is worng. If this were the case, then there would be no way for real property to be loaned or leased.