"I suppose it is tempting, if the only tool you have is a hammer, to treat everything as if it were a nail."
-- Abraham Maslow
That's all this guy did.
Which guy? Do you mean Ms Akram?
Once those particles exited the garden, they were for anyone to collect.
Not lawfully, no. And if you are going to behave unlawfully, why wait till they exit the garden? For that matter why not break in and steal all the silver that is there "for anyone to collect" (unlawfully)?
If the couple wanted exclusive use of those light particles
The couple were not here engaged in physics research where the construct "light particles" might productively be applied. I doubt the couple were minded even to consider the physics of the situation, much less did they want exclusive rights to any particular set of light particles. The couple simply wanted to be able to speak with each other in the privacy of their own home. That, I would submit, is their right.
Your misunderstanding is the result of inappropriately applying a model which is simply impertinent to the situation. The question here is: were the registration requirements and the data protection principles of Data Protection Act 1998 adhered to? That is not a question of physics, but of law. The construct 'light particles' can form no part of any answer to that question.