When I first mentioned the cover your plate thing, I did refer to Asian cultures. As well as what the couple in the OP did would be considered as crass by then. It's about how generous the giver feels, not how much the HC thinks they should get.
With weddings, you can tie yourself in a knot for different countries expectations. For example, I know in the US I know bridal showers are more common and registry details are given then. Here in Australia, they're not as common and registry details are acceptably sent with invites.
I will acknowledge that I am neither Chinese nor Italian. However, I would bet money that in both cultures, although there is a somewhat universal expectation that the guests should want to cover the cost of their plate...it would also be considered rude for the hosts to confront a guest who did not on Facebook, tell them "we did not expect to have to pay for a large part of our reception", make the assumption that they would be invited to the guest couple's future wedding, and flat out state "when that happens we'll give you the same you gave us".
These objections are not insults to anyone's culture.
What the bride did was rude and entitled by almost anyone's standards.
Also, the guest in question stated that she was a recent college graduate who had student loans and at the moment was only working part-time. I have been in her situation except I was somewhat better off because I didn't have any student loan debt, but when all you can find is part time work coughing up $100 is a no small feat! The guest's gift was very generous given her situation. The bride, however, made assumptions about the guest couple's life (you both work and live together, you could afford to give more than $50 each!), and made it clear she felt she deserved more than they gave which, as WillyNilly described it, is not in the spirit of the 'cover your plate' mentality because that should be about how much the guests feel the HC deserves, not how much they feel they deserve.