The atmosphere is composed of about 78% nitrogen, 21% oxygen, and 1% other gases including carbon dioxide. Since plants produce oxygen by photosynthesis which involves them taking in carbon dioxide and releasing oxygen, how could the Amazon be responsible for 20% of the oxygen we breathe?
20% of the 21% oxygen composition of the atmosphere is about 4%. So the Amazon would be producing 4% of the atmospheric composition by using less than the 1% that is CO2 as fuel for its photosynthesis! Unless the Amazon is transmuting elements in nuclear processes it absolutely is not producing 20% of the oxygen we breathe. This is a myth. It actually has almost zero impact on oxygen levels.
What it does do is sink CO2 which reduces the warming impact, and is important in stabilizing rainfall patterns in the region. Plus it is the most biodiverse region on the planet, and home to millions of species.
It should be protected for future generations because we share the planet with other life. Cutting it or burning it down for short term economic gains is shortsighted and Brazil has a responsibility to look after it. If they don't, we can't stop them ... but we don't have to buy products from Brazil if we disagree with their actions.