Dado stacks are not illegal. This comes up regularly.
What is illegal is using an unguarded blade in a commercial environment, or using a blade which fails to stop within 10 seconds.
On many saws, the guard is mounted on the riving knife, and to use a dado stack one has to remove the RK, thereby removing the guard as well.
Also a full dado stack is many times the weight of a normal blade, so the inertia extends the stopping time. On my saw, a full stacks still stops in just over 6 seconds.
But it is perfectly possible to guard in other ways. I have, and use, three guards, none of which rely on the RK for support. Each one is designed for different purposes. Lurker's link to my dado fence has the guard built-in.
When I was filming my DVD series, I did a whole section on how to use dado stacks legally and safely, with input from my local HSE office. They were really helpful, so I do know what I am talking about.
The only problem is finding a saw that will take them. Most euro machines come with a short, 30mm arbor. I have an Xcalibur, which can also take a longer, 5/8" arbor, so I can, and do (occasionally) use a dado stack. For many operations it is quicker to rout, because of the setup time, but sometimes it is worth it for the superb quality of cut.
Using a dado stack is not illegal. Even using one at home without a guard is not illegal (home workshops are not covered by H&S legislation), it's just a very silly and unnecessary thing to do.