Primed TNT fired from a TNT cannon will damage spawn protection

What I would expect to happen:
Blocks under spawn protection would be unaffected by a TNT blast.

What I observed:
A block of TNT, fired using a TNT cannon will damage blocks under spawn protect.

How to reproduce:
Build a TNT cannon and fire it into your spawn area. Non-op players cannot place blocks in this area, but primed TNT fired from a TNT cannon landing in the spawn area will cause damage to blocks.

I asked a player on my server to demonstrate this, and he was able to build a cannon that fired a primed block of TNT into our spawn wall. I observed the wall damaged, and several blocks around the base where the TNT exploded. This was a horizontal cannon, which can be built entirely in survival by non-op players. It consisted of a horizontal channel of water with tnt exploding, which launched a block of primed TNT about 100 blocks into our spawn wall.

Our server is Minecraft1-7.com. The player who demonstrated the cannon for me is BubbleBoyJay. I built a giant bedrock box around our spawn to protect it. It's really really ugly, so it would be great to fix this bug so I can remove it.