“Like other discriminatory legislation in our country's history, immigration laws define and differentiate legal status on the basis of arbitrary attributes. Immigration laws create unequal rights. People who break immigration laws don't cause harm or even potential harm (unlike, for example, drunk driving, which creates the potential for harm even if no accident occurs). Rather, people who break immigration laws do things that are perfectly legal for others, but denied to them--like crossing a border or, even more commonly, simply exist.”