This is the internet.
They are only doing what you would do given the same opportunity.
You probably checked abox inadvertantly giving them permission.
There are free email providers everywhere.
Use one for legit, personal, important, correspondence.
Use another for friends and family
Use another for game Sites and other impersonal stuff in which you can discard at a moments notice.
NEVER under any circumstances use your ISP default email, after all it contains half of your account log in info.
Personally , i prefer gmail for all three. I never use ISP email.
www.gmail.com