It's intimidation and if it interferes with a person's right to free speech it's questionable. The thing is, there's a reasonable justification for photographing people at a demonstration. If things get out of hand it may be difficult to take photos, and the photos that were taken when everything was peaceful might be useful. But it's a whole other thing for the police to study photos of nonviolent protesters and then harass those people when they are going about their daily lives.
We're all still citizens and mutual respect to the degree that is possible, is the way to go. I don't know how others feel, but I personally understand that the police don't have an easy job. I don't hate them, but when they hassle me for no reason I have to say I do see red.
It's strange, because I haven't had any interaction with the police for months, but there was a period, maybe late last year, when I was stopped repeatedly just going to the store at night. It felt like I couldn't leave the house at night without being stopped. I show my ID and try to get it over with. I have no record and I'm not doing anything illegal. But then this one female cop had her little buddy search me, "it's for our safety and we can make you submit to search". I told them I did not consent but I wasn't going to physically resist them. I should have got a lawyer. That's why they have "searchable probation" meaning that a person can be searched any time. I was carrying a bag of groceries. Obviously I was minding my own business.
But when the police harass you and force you to submit to their force, when you are not a criminal and should not have to be treated like one, it ruins the relationship between the police and the community.
Anyway, no, stopping people because they went to a demonstration a week or so before, and treating demonstrators as terrorists, is a violation, and it is completely disrespectful of everything. It disrespects the law, the person, the reason that we have rights, our history and the people that died for our rights, and on and on. This is not upholding the law.