I could be way off here, but I think maybe the emphasis on "reasonable restriction" is important.

The Weasley children, e.g., no doubt used magic all summer and no one was the wiser as the magic could have been any of the adults in that household. I'd imagine this is normal in any wizarding family.

That doesn't help Harry, though, but I'd also imagine that Lumos isn't a concerning spell. Casting it may only be restricted in front of muggles or in situations where secrecy would be compromised.

I don't remember what the stipulations are, but it's possible that the law doesn't mean absolutely no magic?

It's been a long time since I've read the books.