Hi there! First of all, I do see your point, but remember: there are selfish people on both sides. There are selfish brides, and there are also selfish guests!
One specific thing in your post jumped out at me: "How can one not realize that your wedding is not important to anyone but you and your FH...?" Oh, if only this were true! I went into wedding planning assuming that my wedding day would be very special to myself, my fiance, and our parents, but that no one else would really care; that has not been the case. From the day we announced our engagement, we had casual friends/coworkers/distant relatives we weren't planning on inviting saying things like: "I can't wait to come to your wedding," or "When is your wedding so I can request the time off," etc. We've also gotten tons of unsolicited advice about things we must and mustn't do. So, some people really do care about weddings. I've heard of friendships being ruined over not receiving a wedding invitation!
Before I planned a wedding, I used to have a good laugh over bridezilla stories, but now I'm starting to realize that guestzillas exist too. There are some people who look at weddings as a date night for themselves and their SO (or flavor-of-the-week) with a free fancy dinner and an open bar. Although most brides plan the event carefully, considering how best to keep their guests fed, comfortable, happy, and entertained throughout the evening, some guests will still complain and try to make your wedding all about them and their needs. One of my guests -- a 60-year-old woman -- threw a tantrum because I wouldn't allow her to bring a random friend to my 25-person wedding. And I agree that you should never expect your guests to cover the cost of their plate, and that gifts are never required. However, it's generally a nice thing to give your hosts a gift to thank them for their hospitality. And in most cases, you want to give a gift in keeping with the event. For a small casual dinner party, that might be a $20 bottle of wine; at a plated, catered event where the couple is paying per guest, it's nice to give a bit more. So I do understand why brides who assumed they would be getting $100 per guest get frustrated when the guest gives $40 for themselves and the random stranger they brought as a +1.
However, I agree with you that it's completely unacceptable to tell people how to spend their money. If someone wanted to go on vacation instead of attending my wedding, I'd tell them to have a wonderful time. My rule with wedding planning is this: if you'd be willing to move the date for this person so they could come to your wedding, then they are on the "Absolutely Must Invite" list. If it were one of those people -- who blew off my wedding for a vacation -- I'd be pissed. For a random aunt, cousin, friend? Nah. I wouldn't care. For many brides, this is one of the most important days in their life, and they take this as a grievous hurt.