It cannot be pruned because it has to stay around in fast storage to detect double spends, even though ironically it cannot be spent (developers, let me know if this description is technically accurate).
It can _technically_ be spent it's just uneconomical to do it the marginal cost (fees/lost of priority/etc) of including it in a transaction will be less than its worth. If you pruned it and then some smart alec went and spent it (costing himself coin in the process) you'd reject the chain and end up on a fork, which mr. smart alec might find profitable indeed (e.g. if you were a competing miner or a victim he wanted to attack).