Oh really? I'm not an expert on Belgian law here, but are they really claiming they offered to pay people extra and/or give time off in lieu for optional overtime, but they didn't agree to it? Or is it just mandatory free overtime that is banned?
Belgian law probably makes it hard to require employees to work more than reasonable hours for no extra reward or choice. Which is a good thing. If you want 24x7 support, you need to pay for 24x7 employees. This person is wishing they had the right to specify in a contract that your working hours are "whenever I call."
Since they don't get that, I'm not surprised they didn't realise firing someone^W^W, sorry, "agree[ing] to terminate your collaboration" might make them stop responding beyond the call of duty.
Also, I was reading something the other day that pointed out that, whilst seeing things in black and white reduces you to only two perspectives (moral and immoral), the attitude that everything is a shade of grey (and the implied attitude that this makes moral comparisons impossible) reduces you to a single perspective (amoral). If you think two perspectives is too few, how is going down to one perspective supposed to help?