This probably makes it legal, provided that the surfer is over thirteen years of age:
Aside from purely legal issues, Choker mentioned a couple of things related
to pissing off a customer and basically being a scumbag or not.
Obviously if YOU spread your customer's information around so that "the member you
worked so hard to get will soon have his email address rendered unusable due to the
large amounts of spam", the customer is not going to be happy with you, is not going
to keep buying from you, and will rightly decide that you are a scumbag.
Decide for yourself to what extent you want to provide your customers with a quality
service and build customer loyalty versus how much of a scumbag you want to be.
You can then consider what that means you should do as far as the checkbox labeled
"do not share my information", etc.
One company uses "Don't Be Evil" as one of the major guiding principles of how they
do things and that company, Google, has done fairly well. One economist called them
"the most successful company ever". More on Google's ideas about this:
While the value of Google nearly doubled in twelve months, Microsoft has lost value
just about every year for the last ten years. In their "good" years they didn't quite
keep up with inflation. We all know MS is fine with being scumbags. Draw your own conclusions.