It's not a bug, it's a "feature".
Only after people complained about it became a bug.
They tried to pull a fast one and it didn't work. Simple as that.
"...only display their @facebook address on their timeline, the social network didn't intend for it to be the only address that got synced to users' phones."
So hard to believe this one. I mean it is not like they haven't tried pulling a fast-one on their users before and got caught.