The concern is that drones may be dangerous - mostly because they risk crashing on people or objects and causing injury or damage, and partly because of the risk of collision with manned aircraft.
So you can fly as an amateur, with lots of restrictions designed to make things safe, or you can get training and a license and insurance and fly as a professional with less restrictions.
For example, amateurs can't fly over crowds but some (not all) professionals can, if they have a suitable drone with redundant propellers and have done a risk assessment and have appropriate insurance.
As another example, amateurs can't fly near airports but professionals working for some airlines are using drones to visually inspect aircraft on the ground at airports - it's apparently quicker than getting a ladder and moving it around the aircraft.
If you're being paid for your flying, you get classed as a professional and you have to follow the professional rules (training/license/insurance). The thinking behind this rule is that many people who are being paid for flying will be flying more often and in busier places than amateurs do, and also many of them have enough drone-related income to pay for the training/license/insurance. However, enabling YouTube ads is probably not enough to pay the professional training/license/insurance costs, so some people are justifiably unhappy with this rule.
Of course, this doesn't mean that amateurs actually follow the rules.