All rather over the top.
The necessary tip thing is more to do with IMO a piss poor outlook on the relationship between employer and employee in the USoA. No one should abuse folks in need by not paying a decent wage for the job simply because they can, and expecting the charity of customers to make up the difference.
The written query is not that big a thing. It's a simple query as to why they should be expected to tip a waitress a larger amount than they even contribute to their church.
The posting of the note was always going to be a risk but ought not be that big a deal. But we are well aware that bringing one's employer's business into disrepute may have serious consequences. The employer ought to have been more charitable though and supported their employee, giving a verbal warning instead.
The outrage from the customer, as described in the article, is way out of proportion, and probably was requested whist in the initial rage of seeing the query exposed to the world. One would hope after calming down the same demands would no longer be desired.
I don't think something demanded in the heat of the moment ought necessarily be used as a reflection on their general attitude. One might think it could reflect their usual personality, but one has but a single incident to judge from.