I always assumed that a Tie Fighter was airtight until it had a hull breach, by that point having complete separate support is completely advantageous. It also allows the pilot to survive in a very small ship for extended periods of time without investing in ship wide life-support systems when they have their life support on a much more personal system. This saves costs in manufacturing a ship wide life support system and allows the empire to remain a streamlined engine of war.
This is the both ways answer; there is air in the cabin, but probably not enough for prolonged military exercises thus most pilots are seen wearing space suits as standard as both safety and efficiency feature.
The alternative answer is that Rebels is a cartooon and ultimately it will cater to it's primary audience, the children that they want to draw into star wars, they don't really mind the small details and are more about making entertaining viewing. I always took a very simplistic view on canon; I don't really care as long as it's entertaining.
Which happens to be the model modern fighter aircraft use. There is also an advantage to positive pressure masks in high g maneuvers.