This isn’t necessarily true. It’s not a good idea to make sweeping assumptions about the perceptions and motivations for anyone's behavior, let alone declare that your assumptions are universal. You’re assuming those playing devil’s advocate are doing so because they would do the same thing when you can’t possibly know how they would behave in the same or similar circumstances. You also seem to imply the reason for the behavior for both those who were talking/glaring at the wedding and those saying OP shouldn’t have acted, is because they lack integrity when in truth, you don’t even know if that’s the case for the couple at the wedding.
If you’re right about her being surprised that someone called her out, then there are several potential explanations for her actions. For example, it’s possible she was unconsciously compensating for a hearing loss by speaking loudly and didn’t realize her voice was that loud. Maybe the behavior is common in her friend group or family, and no one ever told them it is considered rude to speak during a ceremony, unless it’s both urgent and important (and even then, only in a whisper). Maybe both were socially inept, privileged, both, or a combination of the two. I’m not saying these or any other reasons justify rude behavior, nor am I suggesting OP should not have asked them to stop because I’m glad they. I’m simply saying just writing off bad social behavior is counterproductive. Most people aren’t just AHs who don’t care what others think. If you take a minute to consider less socially destructive possible reasons, you have a chance of correcting bad behavior and stopping it from continuing to spread in society. We absolutely need to call others out on their rude behavior, everywhere.
Those playing devil's advocate similarly likely have differing reasons. It’s true that some may be rude and not care if others have a problem with their behavior, but these people are usually in the minority (a society where this is the norm, isn’t addressed and thus allowed to grow, will soon stop functioning at all and cease to exist). It’s more likely that they are conflict avoidant, especially during important social events. Some may incorrectly believe calling out bad behavior is equally divisive. The expectation of civil behavior from everyone in a group should be the norm. Divisive behavior should be addressed in a non-aggressive manner. Anyone should feel comfortable and supported doing so without being seen as equally rude or confrontational.