Earlier quoted context omitted.
There's significantly less incentive to do that when you don't have to. Or, in other words - why would a Polish worker do that any more than a Dutch person, when both of them are otherwise there entirely legally?
The cheap migrant worker probably doesn't care if he pays the Dutch taxes or not, but his employer will definitely be very happy if he and his coworkers don't. Unreported employment is super common.
http://ec.europa.eu/public_opinion/archives/ebs/ebs_402_en.p...
Employers have every incentive to save that 30%+ by paying their employees in cash.