As someone who IS trained in psychology & a sufferer of severe social anxiety (officially diagnosed) I say this with respect, Highwind - it seems that you are letting it get the better of you & causing yourself undue duress. It's a hard battle, I know - I was shaking after a slight altercation with a woman in a parking lot last week, reviewing the conversation over & over in my head, asking myself if I'd responded with the proper degree of defense/aggression. Intellectually, I knew I did - I took no guff, proved her wrong & we parted ways peacefully. Yet, I found myself wanting to FIND her in that parking lot after my shopping was done & just lay into her - I was cycling through the anger, anxiety & self-doubt for the rest of the day. It happens & it's rough. I promise, I get that & I am sorry that you suffer with this too.
That being said, I feel that a tap on the shoulder to garner your attention in a lively public place isn't really grounds to assume people are irresponsible, disrespectful jerks with no common sense or decency. Sure, it would be better if they were to call out to you with a simple, "Excuse me", but maybe they had already tried that & you didn't hear them? I've had that happen & they better explain themselves AFTER the shoulder tap - "I called out to you, but you didn't seem to hear me, so I'm sorry to bother you like this, but do you know/can you tell me...". Not I or anyone here can say for certainty since we weren't there, but based on how the situation was explained, I see this as a possibility.
Also, look at it this way - maybe they have social anxiety themselves & they were getting that tunnel vision I know I'm prone to & other people with our affliction have confided in me they get when they engage another person in public? By tunnel vision, I mean we're freaking out SO BAD by having to ask someone for something, a stranger no less, so we're channeling everything we have into keeping it together, focusing on that one person we've picked to approach. I've done that plenty when having to seek assistance from someone in public. And hey, I do the shoulder tap too, gentley with my index finger while saying, "Excuse me" again. I do this understanding that not everyone likes being touched - I am very awkward when it happens to me, even just brushing up against someone by accident - so it is a gamble, but I reserve it for after the verbal engagement has failed.
As far as it being against the law - there are MANY laws that are ludicrous, at least without context. In this context, it seemed an innocent approach by someone just looking for minor assistance, no harm intended, so try not to take any.