Because there is a reasonable expectation to the teammates they might be invited. Its reasonable they might not be, sure, but its not out of this world to expect they might.
I guess to me I think, "and so what!" So they didn't get invited to the party. As an adult, why would you waste even a minute contemplating this!? I think if a person is looking to be offended or to play the "poor me" card they will always find ways to do this. Being upset that you didn't get invited to a party makes no sense to me. If you (general) are that offended, scale back your expectations of the friendship if you must. But I don't think it is at all rude for a person to go about her life and not constantly be walking on eggshells that she may inadvertently hurt the feelings of someone by doing so.
While I agree that mature people shouldn't dwell on party invites and whatnot, it does occur to me that the possibility of offense (even if it isn't expressed) by the uninvited is what this particular rule of etiquette is attempting to avoid. I would also argue that there is a point at which it is not unreasonable to be upset that you weren't invited to something without being an SS. If my coworkers all go out to dinner without me I will not be upset because I will generally assume it was a decision of opportunity. If, however, the situation above happened (5 of 6 players on a team go out and no mention to the 6th) to me, my feelings would be hurt. There is something more close knit about a sports team than an office in many situations.
I think my point is - there's nothing wrong with choosing not to invite someone. There is also nothing wrong, in some cases, with that person being hurt that they weren't invited. Thus the rule of etiquette that you don't discuss a party in front of those who weren't invited and who reasonably may have expected to be.
On the general topic, FB is particularly tricky. It could be argued that most people on someone's facebook shouldn't have an expectation of being invited to a wedding, but at the same time, the platform creates an artificial closeness. For example, my husband has a friend who he hasn't seen IRL in 5 years. He has attempted to see her, but she never makes time. They chat on facebook occasionally, but aren't close. About a week before out wedding, a friend made a post about being excited for us and tagged us. The same day, my husband got a message from his friend (a PM) that said "So, I guess I'm not invited to your wedding then". He chose not to respond, in part because he couldn't think of anything nice to say.
Here's my point - the person who made the statement on your friend's page was rude (assuming it was serious). However, even if they were being an SS, it is possible they were genuinely hurt. I'm not encouraging a response - just general compassion. Having just gone through it, I was shocked at how emotional a time our wedding was even for people who were just there as guests. I would encourage your friend to delete the post, not respond, and if the person who posted it brings it up again, perhaps express sympathy with a general tone of "I'm sorry, but it was our day and we got to choose how we wanted to do it - it wasn't about you".