macOh no it isn't
!!!!!!
It used to be that there was a 20Kg weight allowance in the price of the ticket - now there isn't. In just a few years people will read how, back in ancient times, there used to be an inclusive weight allowance (not 'free' just included) and they will be amazed.
Why do carriers adopt this method? Because it works. Everybody wins, because you get what you pay for. Because the human race has collectively decided that money is something to be retained, they are naturally drawn to any process the helps them to retain it. If some individuals land up spending more than they planned - they can choose to amend their behaviour next time.
Times have already changed and you can change or not, as you wish.