(this is usually not enforcable unless you actively build competing services)
What I normally do is ask for an agreement that projects unrelated to my employer's business is ok for me to work on, which I have always been able to get (sometimes they ask for authorization on a case-by-case basis, but usually it's just common sense).
Does it really? I am from Europe and I have lived and worked in 3 EU countries, many of my friends have experience from other countries and I have never heard anyone mention this is. Obviously, my personal experience is limited. In what country do you live if I might ask?
You also bring experience and knowledge to your employer that may benefit them while you're on holiday, or off sick, or even after you leave the company. Maybe we should send them a bill for these incidental benefits?
And European law is just the same.