[coolaliasbro]

It's not mean to express boundaries. Strangers offering unsolicited advice, regardless of intent, crosses a boundary for many people. And since the employee making the comment would never be personally liable in the case of a lawsuit, such advice boils down to misplaced concern or simple nosiness/bossiness. People should mind their own business unless their is a clear and present danger to thselves or the public. A kid horsing around on a hauling cart does not match that description IMO.

[chrisseaton]

Yes in the street. But you’re in their shop, and the horsing around is being done in their property on their equipment. You're a guest there. Either leave or politely comply with what they ask.

[coolaliasbro]

It is extremely unlikely in this case that the employee was also the proprietor, so there is no "their". It was an individual decision, not a business decision.