The problem for these companies is
are they charlatans, or are they indeed going to carry out the attack if you don't pay.
A mugger may pull a knife on you and say they are going to stab you if you don't hand the money over. Just because you hand the money over and survived, doesn't mean they weren't going to stab you.
However, some of the money may be spent more wisely on DDoS mitigation, rather than paying the mob.