There you go again... that impressive psychic ability that allows you to perceive more than the average bear. Could it be that you just are one of the LUCKY ones that got good instructors that didnt screw you?
Exactly, how does a complete beginner knowing nothing about martial arts tell a good instructor from a bad one? Even the most astute have been caught out by instructors who have ripped them off, it usually takes a meeting or conversation with a martial artist who trains under someone else, often on here, before people realise that their instructor isn't all he/she should be. In the case of withholding certificates, everything may have been fine up till that point however, taking money for something and then not handing over that something is fraud.