| I wonder why, especially in this data stealing age, the
practice is not firmly
| against the law?
Yes. And the boundaries clearly marked.
The problem is that its a matter of private law.
The government would essentially fetter itself if it actually
made it illegal for you to give out your SIN voluntarily. This
might be the case in settlement if someone has sued you, won
and now has the right to a full accounting of your income and
assets.
Enforcing laws is expensive and there is a
threshold which is bounded by economy of scale. As a general
matter of private law, caveat emptor (let the buyer beware) is
the rule.
Its kind of like the government is a national
park with a grand canyon running through it. The can put up
signs which say don't get too close to the edge or you may
fall in but they can't really stop you from jumping off the
edge.