As long as the store did not discriminate based on race, religion, creed, etc it had every right to refuse service for any other reason. It may be bad customer service, but nothing criminal to refuse to serve the man.
Once the cashier asked the man to leave and he didn’t that became trespassing and it is perfectly legal in Texas to remove a trespasser by force. The security guard acted injudiciously but not illegally when he grabbed the man.
Personally I have two concerns here. First is that the off-duty cop’s first instinct was to manhandle an unruly customer which I have to figure is learned behavior from his time on the force. Second it worries me that the DA feels he can charge assault on a public servant even though the off duty cop was acting in a private capacity, as though mere membership in the police club makes you more important that the rest of the population.
Were you there? How do you know the cop just went up and grabbed the guy? How do you know they didn't have a screaming match first and the suspect said, "I ain't goin no where!"
You are assuming that was his first instinct. The news report simply isn't saying, isn't providing any information or details regarding what happened other than to paint the store and officer as aggressively insane.
In many jurisdictions police are hired out for private security work through the department and are paid through the department but reimbursed by the business. When they are off duty/working outside employment they wear their uniforms, carry their service weapons and also have their police radios and login with dispatch when they are working. It is a very blurred line.
If it was evident that this person was an officer by his uniform and he was assaulted anyway, I think there may be a case.
You are the only one with a reasoned response for the most part. Unfortunately without a video I would never believe the cop for any reason whatsoever. Pretty sad, but I have seen too much in my life to ever have faith in the police anymore to be honest ever.