Arena cap for gold bannable?

[Snuk]

On MoP realm, asked for 2 wins on global for gold and a gm contacts me with the following message:
"Hello xxx, buying arena boosting is not allowed on our realms. Please avoid asking to buy it. This is your only warning and next time your account will be suspended"
Since when paying with IN-GAME currency for IN-GAME benefits is bannable?

[Obnoxious]

If you have an issue with a GM ruling, feel free to send one of them a PM in the Forums to ask for clarification.
The Forums aren't a place for you to open threads and come question their decisions.