Being a Mercenary shouldn't be a bannable offense.
Being a mercenary is just like offering any other type of service to characters in Eorzea and makes sense. Trading services done in FFXIV for Gil, an FFXIV currency, is no different than paying gil to have someone meld gear for you, or buying a person's crafted item from the market. No its not an official 'job' and may be looked down upon by a few, but being a merc is a legit way of making gil, unlike trading real money for digital currency which could easily lead to password theft and other serious issues.