Cannon minion bug?

I was playing a game recently and after we took Baron, I buffed my top lane's siege minion with Banner of Command. I noticed him start attacking very slowly, so I went to some custom games to try to find out why. Two custom games later (because I didn't think I could kill Baron alone with nasus.) I found out that Baron buff was slowing his attack speed. I'm not sure if it's a bug or not, but when a cannon minion has baron buff on him, he attacks at half speed (1.00 to .50)

The banner of command had it's usual buff effect, and was irrelevant to the attack speed.

Is this intended? I know they gain a lot of range when under the baron buff as well, so I could see it for some balance reason.