How did Colorado Shooter get Tear Gas?

I was asked by fellow co workers wouldn't it be difficult for someone to get tear gas?  Unfortunately it's not as you can see at http://www.keepshooting.com/clear-out-2oz-tear-gas-grenade.html

For $10 you can buy a tear gas grenade if you are 21 years of age?  The tear gas grenade contains 2oz of military grade OC/CS in a 1%/1% solution which will deliver a fog to incapacitate a large crowd.  The 2oz. units disperses in 8-10 seconds and covers an area approximately 9300 cubic feet in 30 seconds.

It's troubling to me that as long as you are over 21 years of age you can buy tear gas grenades it appears.  The site doesn't say whether they do background checks or not.