Back in 1.8, players were able to change their own spawners with eggs, so when we started giving them out, we knew we couldn't give anything OP. When 1.9 rolled around, it got patched so that only creative players could change spawners. We started changing spawners for the players as we did advertise the fact that eggs could be turned into spawners. When shulkers first came out, they were useless, it seemed harmless to make them an egg. Now shulkers are arguably op and the fact that we have shulker spawners is even more so.
Going forward, we will not be changing spawners for anyone, it's not Mojangs intention, takes up staff's valuable time, and creates an imbalance in the economy. I realize some players are holding on to their eggs for a later date so I will give notice in advance that you will not be able to place your eggs into spawners coming next month.