Yep, that is a result of the laws of physics. Without an air vent, you would neither be able to fillit OR empty it. In order to pump fluid in,you will need to diplace the air inside of it. In order to create a vaccuum to remove the fluid you will need to replace the fluid with air as well. Otherwise, you couldn't fill it when you flushed and you would crush the tank when you sucked the contents out. don't worry it is working as designed.
While discussing vents. Consider installing an inline vent filter. Dometic makes a nice one. It keeps the odor in the tank when it is displacing the air when yo uare flushing. This way you're not sharing with others.