I think you're missing the point. The North Wales Police are using the tactic of stopping motorcyclists indiscriminately using part of the road traffic act which other forces barely use - as in they can "stop any mechanically propelled vehicle". If you watch "Traffic Cops" on BBC iplayer, you'll see that even when they have a tip off saying someone has been drinking and driving, they still have to have a reason to pull the driver over (in this case not signalling and not wearing a seat belt) because whilst the law allows them to do what they like, policing in the UK is about engaging with a community not clamping down on it.
Their tactic is to waste people's time, intimidate them and generally put the word round that North Wales is not accepting of motorcyclists. Yes there are idiots among us but frankly the same is true of car drivers, the difference being that idiot motorcyclists tend to kill themselves and make statistics.
You'll find that whilst people take issue with the tactics being used, they do not blame the individual officers as we are well aware they are doing the job they are told to do. Read the advice being issued about what to do if stopped and you'll see it says be polite. Documenting things, asking for PACE, etc is a way of countering the tactics being used to ensure the officer is tied up for as long as possible as therefore can not stop anyone else in that time. We're not trying to piss people off, we're making a stand against a tactic which is completely against the spirit of modern day policing. You'll find a lot of small businesses which survive party on the basis of passing trade from motorcyclists through summer hence why the locals will, in general, be supportive. I do expect the police to unjustly use anti-terror laws like they do with many protests to identify and record those people who attend - I will be very suprised if they don't.
Yes I totally agree that if you want to use the roads you should be legal - it's the law of the land and if you don't like the conditions set down in the law under which you may engage in whatever activity, then don't do it or go elsewhere. Illegal plates are illegal - full stop. The impact on ANPR means your bike is less likely to be picked up if stolen, means it's harder to read for police if you break the law and frankly those loud pipes just annoy everyone. If I want to be heard whilst filtering, I knock it down a gear - it's not safety it's about general politeness and not being antisocial.