EXCELLENT point! ^ It's certainly not documented, and it is not even an undocumented benefit - it is a very nice gesture a number of AA CSRs make, most likely because it is not strictly prohibited by AA and costs AA minimally.
Let's start with the main fallacy here - entitlement. The entitlement is for 4 stickers at each 10K miles. That's it. These partial sticker gifts are a favor, not an entitlement. There is nothing in any written policy that indicates anything close to an entitlement.
I'd still cause since it can't hurt. But definitely don't look at this as any sort of entitlement.
Cheers.