We never knew who they were before. Now we will. Those who apply will have written record and their employer will be responsible for reporting if they become unemployed. Those who do not have blue cards within a certain timeframe will be deported automatically. All the lobbying and all the lawyers in the world will not be able to stop it.
That, my friend, is the difference. Our current laws have been ignored so long that lawyers and activist groups get these people off. With new rules in place, they can't say "this is the way it's always done".