AppleTV Remote Controlling iTunes on Laptop
This is because they both use similar technology. You need to pair the remote with the device you wish to use it on if you have multiple devices. 

You can do this on the Apple TV by:


going to Settings
General 
Remotes 
Pair Remote


More info is available in the Setup Guide as well.

If you want to pair it with the Mac, you can do the following steps (From an Apple KB):


Log in on your Mac as an Admin user.
Get very close to your Mac (about 3 or 4 inches away).
Point the remote at the infrared sensor on the front of your Mac.
Press and hold the Menu and Next/Fast-forward buttons simultaneously on the remote for 5 seconds.
Your Apple Remote is now paired with your computer. You should see a chain-link style lock appear in the center of your screen for a moment.


If you never want the Mac to be controled with the remote, then you can:


Go to System Preferences
Go to Security 
Select  "Disable remote control infrared receiver"