If that were the UK they'd put that Briton on an antiterrorism watchlist
You made it sound like he was put on the watchlist for his good deed on London Bridge. That article is sparse of any real detail, but still says "Anti-terror cops first got in touch with him amid footage of him yelling abuse at a black man while walking his dogs." Which (for reasons not fully revealed) led to him being watchlisted.
Let's spin some theories, in leiu of knowing very much about him (meaning that I'm being entirely hypothetical here):
1) He's a bad-egg all round, and close to wanting to thump somebody 'furrin' at the time of the terror attack, so he was the right person at the right time regardless of it being for bad reasons.
2) He
wasn't (counter-)radicalised - and not every Millwall fan (or most, even?) is a headcase, so throw that other reported fact out - but it's reported that he attracted the attention(/admiration) of those that do and, through their reaching out to him, liked their more extreme biases. This leads him to be the racist-radical who can't be given a pass just due to past good actions.
3) He doesn't even have anything against muslims, the bridge incident was entirely altrusitic and he rejected EDL/whoever with their anti-islamic basis, but he's just plain anti-black enough (together with any personal animosity towards the individual concerned[1]) to cross the threshold upon investigating this incident, and maybe any others taken into account.
Though the "police are useless" line is based upon highlighted cases, not the whole system (and we can argue the scope and degree until a month next Saturday, and still not agree), I'll add the following
anyway:
4) He was minding his own business, it's a pure fix-up, everything was taken out of context and at every stage of the process the system managed to ignore the fact that he's being persecuted by the police, CPS, the anti-terror squad, the local society of corner-shop propriators, The Kennel Club (for having no dogs/the wrong dogs), the late Paul O'Grady, West Ham United and/or George Soros.
There are indeed problems out there. Though I could point at issues with the Anglican church, to counter any too-forgiving attitude to those ethnically islamic (anti-anti-Muslim?)... Apart from being yet another horrible-crashy newspaper site (luckily, few enough words to battle through, before it went kablooie on me), that article doesn't necessarily record any injustice. If you know more about it, you've failed to convey the salient details.
[1] Was going to suggest also against dog-walkers/dogs, but now not too sure whether the above means he shouted at a black man walking dogs (were the dogs a problem?) or, whilst walking his own dogs (again, were the dogs a problem/agrevating circumstance?) he shouted at a black man. Lazy newspaperese writing, such that I don't even know which situation to work with. So I won't do either.