A thieves'guild legal?? Nah...
Underground, well-known 'secret' organzation, yes. Calling themselves a guild legitamizes them - not at all. Remember, Capone's organization/business sounded legit, as well. I think too many are hung up on the word "guild". Seems likely to me this is what such an organization might call itself - but just because the sign outside my establisment reads "Flowers by Cathbad", doesn't mean I'm not making counterfeit money in the back room...
Do you think the Cosa Nostra hung out signs saying, "Guild of Murderers, Thieves and Influence Peddlers"?