Because the Republic had laws and banned several of the Cartel's actions such as slavery, trade of certain goods, trade rights, and tax evasion... yadda yadda. They are criminals and the Republic is a governing body with galactic force capable of ensuring stability and maintaining their laws. So it seems very appropriate that the subject of authority would be seen as the enemy of the Hutt Cartels as I'm sure the Repubic Senate, not to mention the jedi, were bad for business. Also Republic armor is a decent armor that is just as durable as most medium grade armor is. Therefore, practicing on them grants a level of realism to how much dmg their blasters can do and makes for a more enjoyable target than a poster or a non-colored suit of armor.