Imagine a world where they allow Pebble to go through certification process for it to get jailbroken half a year down the road opening the gateway to iMessage for all the spammers in the world. What then? Should Apple now play whac-a-moll with the spammers forever, or block the access to all Pebble watches creating another scandal? And what if this happens to next 10 different watch makers down the road?
They own Apple Watch and if it gets jailbroken its their mess to deal with, but if they open it to the world then they have zero control over it.