It's down to the ethics of the company. I'm sure some would lock just to make it awkward to move maintainer.
There are genuine reasons to lock panels from the installers POV tho:-
You are protecting the company from a malicious claim, say if the system failed to operate because it had been modified by an end user or another maintainer.
Making sure the system is maintained as per any out of hours agreement and that expiry of this is displayed on the system.
Maybe the system hardware isn't fully owned by the customer, it could be leased or yet to be paid off in full.
There is also a possible argument that the programming of the system is the intellectual property of the maintainer, even if you own the hardware.