Does the tinfoil not make your ears itch?
All the spray does is prove you were in a particular location at a given time. Every other element of burden of proof that you committed a crime needs to apply.
You may have noticed that certain sections of society figured out during Covid times that wearing a mask makes you harder to recognise.
If the police had witnessed or held video evidence of a crime being committed by a mask wearing nerdowell, this spray can be used to prove who that was.
Approximately 2% of the population have a skill known as super recognition. The police use those with this skill to review video footage. So basically, whilst you might be wearing a mask, you can still expect a knock on your door if you’re recognised. And if you’ve been sprayed, you can’t deny being present.