The thieves' guild are not actually the Welcomers, nor were they ever thieves. Originally.
While they have been managing the affairs of thieves and have even gained notoriety and respect from other guilds, they are in fact in search of another source of riches—a dragon graveyard. Years ago, when the Dragon Run of 1306 Dale Reckoning tracked across the Moonsea, nearly obliterating the old city, many dragons were rumored to have fallen.
Having swallowed their treasure horde, they died with phenomenal wealth in their bellies, literally many tons of gold, silver, and precious stones, just waiting for someone to take.
The thieves' guild is now two tiers, one of actual thieves who have no idea what the core leaders of the guild are truly after, and a core guild of officers who know their true purpose, and intent. They are well connected, and they have been well paid to obscure their efforts.
For the time being, the thieves have agreed to stay away from the city, although a small element of their number will continue to raid newcomers to the city, as they historically have for years.