I have no clue as to how the players felt, but if they didn't want to be there then why did they show up? It was off season. The team can't retaliate against them otherwise the players union would be up in arms. Athletes go to these things because they want to do, or the don't go because the don't want to go. Nobody holds a gun to their head either way.
Standard MLB player contracts do indeed specify that players participate in team promotional activities.
I believe MLB is stricter about required appearances than other pro sports.
Not surprisingly, the bigger the star the more likely he can get away with snubbing team activities.