In what country can an employer dictate what you do in your own time? Just because an employer puts somethings in a contract doesn't make it legal.
I understand non-compete clauses, but this doesn't seem to apply here.
In the German system the oral addition stands above the written contract. The written contract is considered boiler-plate by the courts and the oral addition is more specific, and bears more weight.
Of course all parties have to agree to amend a contract. So just the employer changing their mind wouldn't cut it.
The Netherlands is a civil law country, so the German standard might be closer to what they have than the common law American standard.
Labour relations and law are of course a strange thing in themselves.
You've wisely chosen to not ruffle any more feathers there. Just behave and get out when you can.
In many cases when an employer wants to get rid of an employee for a reason that isn't covered by the employment contract he has to request permission from a local judge (small claims court equivalent)... and even then the judge can determine an outrageous "termination fee".
So although the rules like these look draconian, they're sort of needed to keep the power between employer and employee in balance.
In Canada this practice is not legally supported, but still relatively common.
Maybe I'm paranoid, but just to cover my ass, these days any time I agree to anything with just about anyone, I insist on adding an explicit clause that makes it clear that I am free to work on any other project with no restrictions (I specifically retain the right to compete), and that except for the works specifically covered by the contract, the other party has no claim or license to any intellectual property that I own or create.
Then again, that's par for the course when it comes to work-for-hire, it's been a long time since I've signed an employment contract as an employee, and I doubt those additions would fly as easily in that circumstance...still always worth a shot, though, companies will often agree to change details of contracts for you as long as they don't feel they're getting screwed by the changes.