I think "illegally installing software" is a red herring.

Rather, the issue is "Spy Camera Artist." It's possible that most of the folks photographed for his show have no idea; and might not be happy.

While photography in a public place is protected, private property photography is at the discretion of the property owner.

Apple might come to the idea that unwittingly photographed customers could sue them for invasion of privacy. Hence, they went after him.

There are plenty of other reasons and explanations as well.