It seems that whenever you buy something on the Internet this leads to more email spam in your mailbox.
However, a lot of legitimate companies are starting to recognize that selling your email address to someone or even spamming them themselves without your consent gets them in trouble.
However instead of being very overt and forthcoming about getting your consent they are being subtle and many people miss the opportunity to say no because it is so hidden.
Whenever you buy something on-line read the entirety of all the pages of the checkout process. Especially look for a little title checkbox that is you either have to check to not be on their mailing list or uncheck. Sometimes there is more than one: