There’s an entirely 'nother situation where seeming unaffected parties gain standing, not yet mentioned in this thread that I can see.
IANAL (and never regretted it), so I can’t claim to have all the details quite right, but here are the basics:
Some laws appear to be written that explicitly give standing to parties that are otherwise uninvolved. This seems to happen a lot in civil rights legislation. I don’t know if this happens much in Federal law, or much in many State laws. It seems to happen here in California, at least.
You may recall, some years ago, there was a big “Political Correctness” stink about the words that real estate and rental advertisers could use in their advertisements. It was (and is) illegal to discriminate, e.g, “Whites only need apply” or “Irish/Jews/Blacks/whatever need not apply”. It got crazy: If you wrote “Near schools”, you were “steering” childless couples away; if you wrote “Great ocean views” you were discriminating in favor of the sighted and against the blind; if you wrote “Walking distance to shopping/wherever” you were discriminating in favor the the abled and against the disabled. Real estate lawyers published big lists of “forbidden” words (in red); “use with caution words” (in yellow); and “safe words” (in green) – I used to have such a list, on a laminated plastic card.
(ETA: IIRC, this was a nationwide phenomenon, not just in California. And I don’t know if the laws explicitly made anyone-and-everyone a plaintiff with standing, or if the courts “discovered” that in the concept that the laws were for the benefit of all society and not just for the benefit of aggrieved individuals.)
Okay, so what about the question in this thread? There arose cadres of self-appointed civil-rights arm-chair lawyers (who may have been actual lawyers) who scoured real estate and rental ads in the classified sections looking for forbidden words, and filing lawsuits (some would say, nuisance lawsuits) against violations that they found. The suits may have been mostly junk, but they cleaned up by collecting settlement money from defendants who found it cheaper to settle than fight the suits.
And where did those cadres of self-appointed civil-rights lawyers get their standing? Because somewhere in some anti-discrimination laws, it said that anyone could bring those suits on behalf of the “public good”. You see, discrimination wasn’t held to be hurtful only to those individuals who were discriminated against, but rather, was held to be hurtful to civil society in general. Thus, anyone and everyone had standing.
Here in Stanislaus County, CA., there has been something very similar going on for several years now. They file ADA violation lawsuits by the hundreds (often with an actual disabled shill as the named plaintiff, so maybe not exactly what OP is questioning); often at places those alleged plaintiffs have never even visited; often over trivialities.
Anyway, here are three articles I just googled up on the subject, two in Stanislaus or surrounding counties; one in Bay Area counties:
Wave of disability lawsuits threatens small businesses in Stanislaus County
Lawsuits fail to satisfy either side of ADA coin
Serial ADA lawsuit filer striking Bay Area
I also noticed a few headlines that seem to suggest that, at long last, the state legislature and governor are taking some baby steps to crack down on the problem:
Governor signs bill to limit lawsuits, allow ADA fixes
Now, in these ADA cases, it seems that the plaintiff’s lawyers actually have a named disabled plaintiff, but they are claimed to be just shills. Now, I wonder if a lot of those cases with ACLU, SPLC, etc., might work similarly: The organization actively recruiting aggrieved plaintiffs, possibly in mass production numbers, to support their suits. To be sure, flaming liberal that I am, that’s fine with me (well, sort of) when ACLU or SPLC does it – but if that’s true (is it?) it does seem sleazy.