Reusing AN bottles

AN doesn't recommend reusing their empty canisters or bottles. What's the deal here? I'd like to keep and use the old 1-liter bottles since they are so convenient to handle, while the larger containers are cheaper per liter to buy.

I believe the only reason they say that might be because of mold or residue
It also can be thought of (By some conspiracy nuts) that it's an marketing scheme for you to buy more AN.

lol, I've heard of not drinking from reused plastic waterbottles but wtf?