It's exactly to avoid this sort of exploitation. A lot of people don't seem to know about labor protections, unfortunately employees are just as misinformed of their rights as business owners are.
Create buffer companies for those contractors to also work for indirectly for you if necessary. You might still have to pay some taxes, but you won't have the same problem as the Swiss salary cap is trying to prevent if it were implemented.
There will always be ways to exploit systems. You see it as exploitation, but those running the companies see it as necessary to compete.
Also, parts of contracts can be invalidated. Just because you put something like that in the contract doesn't mean you're not violating labor laws.
Some jobs make more sense to be outsourced, in which case contractors will usually already exist and offer "reasonable" prices that will be cheaper than having a full-time employee around.
Other times it doesn't make sense, but employers will try and convince employees to become contractors, so that a lot of employee taxes end up coming out of the employee's side of things. Of course, the employee thinks that this is necessary to keep their job. The construction and taxi industries are very bad at this. It's exploitation, plain and simple.
In some countries (I think UK is like this), they will actually look at the amount of work a "contracter" does for a client. Do too much (i.e. only work for them) and they (the taxman) treat you as an employee. Doesn't matter what you put in the contract.
It's not nearly that simple.