Why curse them?

Because Catullus is an invective poet-- and that means Catullus is always ready to attack!!  Think of Catullus as an improv comic or a performance poet at a poetry slam-- his poems are performance pieces, often in response to events, people, and other poems we don't have available & so can't understand, and often using comic stereotypes that, again, we don't quite grasp.  Egnatius, the smiling Spaniard who brushes his teeth with urine, is a good example of "over the top" satirical invective-- he reminds me of Kenneth in 30 Rock.