I'll tell you why. Because by the very nature of a 'silence', any heckling it ends up as the only voice by default, and leaves those wanting to show respect unable to answer back without further disruption. Making noise during a respectful silence is the 'protest' of a coward. That's why.
Oh, and because it's for someone who has recently passed away, whoever that was. If your mother or wife passed away next week, would you appreciate it if some guys who didn't like her showed up at the crematorium and heckled while the coffin went through the curtains, and damn the feelings of you and yours?
This whole thing is NOTHING to do with being 'royalist' or not, and EVERYTHING to do with basic human decency
If you can't see that, then it's your problem. I personally couldn't stand Diana, for example, but I wouldn't have jeered during a silence. Consideration for others' viewpoints, and also the good name of the club, cost nothing.
But I appreciate I'm probably talking into a vacuum here, as I'll be accused of being a serf or whatever. For the record, I had no issue with the Queen, but I always liked Charles more, if only because he actually cares about the planet, and at least tries to use his position to encourage change, but that's a different matter.
Embarrassingly Kopite behaviour, some of this.