I vaguely recall that the liability laws regarding property owners draw a distinction between simple trespassers on the property and "invitees"; that is, people who have been invited to be on the property.
In this case the customer taking the video starts out as an invitee. He has every reason to believe that he is welcome to engage in a transaction with the business. I'm not sure what legal implications this might have, but it might burden the business with duties they wouldn't have if the person they are trying to eject entered the store during non-business hours, for example.
Similarly, in contract law, an offer to sell followed by an acceptance of the offer constitutes a contract. Does the display of merchandise with marked prices constitute an offer to sell? Does asking to purchase a marked item constitute an acceptance of an offer? Perhaps the circumstances in the video depict a verbal contract between the parties and the clerk was attempting to violate the terms of that contract.
I can see possibilities for this situation to be other than just a trespassing incident.
Do I understand that the clerk has been fired? It might be interesting to know what the owner's intentions are with respect to the customer. Even if the clerk was acting outside his delegated authority, there could be liability for the owner if the clerk was not adequately trained.
We live in interesting times.
The clerk was 100% jerk-wad millennial.
Earned a night at the gray-bar if you ask me.
Swing at me and I probably get locked up for breaking his nose.