I think the cake smashing is a terrible and disrespectful, and yes as Abby says an immature thing to do. I don't ever voice this opinion at weddings or really much at all unless asked, but I do think it.
And as a guest who is giving of my time and energy to attend the wedding, and dress up and look nice, and give a gift, etc, I do think its my business if I am asked to witness this act of public disrespect to a union I was just asked to witness as a public act of love & commitment.
I think the letter writer has a point - when you ask people to attend your wedding you have an obligation to them. You are obligated to actually provide whatever you have offered to host (cake & punch, dinner, whatever), you are obligated to at minimum say "hi" to each guest, and I think you are obligated to publicly honor the vows you just made. There's a time & a place for joking disrespect and I personally don't think at the wedding is the time.