I guess it translates into the lack of political will to curb the practice, too, as the government would risk that self-employed programmers (and whatnot) may not fall in line, but mostly log out of the system instead, registering their companies abroad, for example. I guess the government isn't eager to start this cat-and-mouse game, preferring a smaller slice of a bigger pie.
There are even some extra incentives on the top of the flat income tax rate (which is an option for all one-man companies, regardless of the sector)...
Like the "IP Box" tax relief, which drives the income tax rate from the regular 19% all the way down to 5%, as long as you get your services classified as "research and development". It takes some patience and loads of legal paperwork, which you obviously have to pay for (the latter, that is), but it's well worth it in the end.