Thief is classified as an Adventurer compare to Warrior and Guardian who are classified as Soldiers.
So what does an Adventurer do exactly?
Adventuring is daring the unknown and discovering new things. That’s why Engineer loves their tools and inventions; and Rangers loves their pets. Adventuring is also taking risk for a great reward and that’s why Thieves steals.
Typically when Thieves are caught, they face punishment and in some culture they are not only hit by a nerf bat, they also faced with a dismemberment — an act of making sure that the Thief will never steal again.
So how does that reflect the Thief in GW2?
Thieves in GW2 are risk takers. Players who plays Thief takes the risk of getting hit by a nerf bat because they are playing the profession really well — and Anet plays the barbaric culture of dismembering the Thief so that whenever we steal, we get cut into pieces because Anet wants to dismembered us from our ability to regain endurance.
So the Thieves in Tyria started to sign-up for military duty to become Soldiers and many have joined the ranks of Warriors and Guardians.
In Tyria, stealing and necromancy are dishonorable professions, thus they get hit with the bat more often that the “goody-two-shoes” professions.