Various ideas such as 1) every person you have met including inlaws must be a bridesmaid or they will never speak to you again 2) every person you have ever met must be invited to the wedding or the relationship will be severed 3) people must have a role and just being a guest watching/enjoying is an insult.
I have attended many weddings and never come across of the above situations but they seem to be the norm on WW and supposedly in the current society. Do people really think this is normal for folks to be offended because none of them are rude at all. If anything, it's more rude to assume the opposite: that you must include everyone no matter how unrealistic and stressful it may be on the couple or the others involved. Many people do not want the emotional/financial stress of being a bridesmaid or any other "role" to fill a spot. Many don't care about the wedding of someone they haven't spoken to in years or that they don't click with. Being a guest only is what many prefer. I would imagine planning would be less stressful when couples realize that more people than not will not be offended unless something is truly a breach of etiquette.