Wouldnt an airlock inhibit aeration of your starter? I guess that is why some people use large vessels? It seems contradictory to me to use an airlock on a stirplate that is trying to aerate while limiting the amount of "aer"
Someone can correct me if I'm wrong but I believe the idea with a yeast starter is that you are introducing such a relatively large amount of yeast to a small amount of wort that nothing bad really has a chance to infect it. As opposed to using a relatively small amount of yeast with a full five gallon batch where it will take awhile for the yeast to multiply and consume all the sugar leaving it exposed to the baddies.
Therefore, using aluminum foil is acceptable and even advisable because it allows the starter to aerate better meaning the yeast can grow faster and stronger.