I believe a company should be allowed to market to customers for a limited time after a purchase with any information the customer provides them, UNLESS the customer opts-out. The act of purchasing, providing your email, CC#, address, etc, is opting in to some marketing, and I think it's perfectly ethical as long as it's restrained and infrequent. (That's why I think the monthly or semimonthly newsletter is a good format.) If it isn't, it'll be counterproductive for the company when everyone unsubscribes/blocks/ignores them anyway.

Most sites will give you an option immediately to opt out of newsletters and information sharing, a few will simply tell you they will do these things, some just won't say anything. I think the first should be mandatory, but until it is, the best you can do is weigh your need to buy from them against possible future spamming if they don't offer it.

If you can't find a communication policy and no opt-out is offered on sign-up, it's safer to not trust them and use a dummy account or skip them entirely for a site that gives you more control. You have every right to avoid untrusted merchants and sites, the way I've avoided signing up to myspace because their policies are so scary.