I’d just like to add that the only consequence I see of making crops and tilled farmland siegeable will be to encourage griefing.
Crops can’t prevent you from reaching a player you are trying to siege, and they can’t be used to protect chests.
Unless I am unaware of a server-wide pandemic of players locking their chests and barricading themselves behind tilled farmland, I would argue for these particular blocks to remain unsiegeable.