Browsing is entirely different from spamming. With browsing, a store (online or offline) has opened its doors and said "Please come in! Look around, see if you want to buy something!" And we do. It's understood in this model that we are not obligated to buy something. We are browsing.

Email is a means of personal communication. I trust that when I give out my email, it will only be used for the promised purposes - such as giving me order receipts, or establishing an account. Giving out this email for one purpose is not implicit permission to use it for another purpose.