Thats exactly whats been happening.
When an illegal is caught with an expired visa for instance, he can be deported because he voluntarily signed an agreement (or contract) with the federal government to leave within an allotted time. Since he did not leave, he has violated the terms of the contract.
When an illegal is undocumented, there is no contract. With no evidence to the contrary, the government must assume him to be a national of our country by default and cannot deport him. So he is free to stay in this country for the rest of his life.