From an article on ESPN's Soccernet (note the part in bold):

Inter were also banished from San Siro... for two games by Italian officials later in the same season after their supporters threw a motor scooter from the second tier during a league game against Atalanta.

These people are true hooligans. Chanting? Beneath them. Throwing batteries, coins, food? Child's play. Throwing a motor scooter from field level? Right direction, but they wanted to go bigger.

Perhaps stadium security was busy? I assume the motor scooter from the balcony didn't start the riot, so they may have had their mind on the hecklers and battery throwers (aka "amateur hooligans") while the scooter was making it's way up to the second level. I think the security folks should shoulder some of the burden here. Do they think Inter Milan fans are the only ones capable of throwing motor scooters?