Thats like saying a driving instructor doesn't want their students to pass the driving test.
There are people out there who enjoy their jobs and want to see people acheive their goals. And if their clients acheive them then they as a PT have accomplished something too. Great, move on to the next one.
If they are half smart then they will be setting their clients the next challenge. When they've acheived that: next challenge. Next PB, next distance record.
Though I concede there will probably be a minority of cowboys out there milking people for all they are worth.