If they use this reason, they must reduce total tickets available for sales in this match, like if it is a 16.67% reduction (500 from 3,000 tickets), they must do the same with total tickets available for Arsenal fans in this match. There are no information that Arsenal are doing this on their fans but only want to do this on PSG fans. Generally it is an unfair treatment and ticket allocation from Arsenal for their guest, PSG in this important match for both clubs.
Reason can be used but it must be applied fairly for both clubs, not only on PSG while Arsenal are not affected. If this continue to be a final decision for a first leg semi final match, Arsenal won't deserve to win this match and even if they win it, it won't be a convincing win for them.
I am not entirely sure what 500 people would matter, so I have no idea why Arsenal and PSG are arguing over such a petty thing. If there is a London marathon or whatever, let PSG have the full 3k and drop it from Arsenal part, they would still have 57k or so, even if they have just 55k, that is not going to matter too much.
In any case, this is a petty discussion and it is such a shameful thing that they are both arguing over it. Arsenal is the one that started it so they are in the wrong even more than PSG, why would you do something like this when you know very well that you could give PSG what they want. Or like you said they could just lower the total number, that way %5 would be 2.5k tickets and that would be fine too.