He didn't have the money taken because he was illegal, he had the money taken because he didn't declare it. The seizure rules allow you to take people's property without a finding of guilt or a trial on the merits, through a simple declaration with judicial review, like a warrant.
Being illegal, he will be deported. But being illegal had nothing to do with the money being taken from him.
Being Illegal had everything to do with him not reporting it. You said that yourself! Can you see yet that you cannot seperate out, and ignore, the fact he was an illegal alien and what that means to the overall picture?