Opt-ins should be required for everything, not assumed.
Signing up for something does not entitle a company to send you email later on, unless you opt-in. Most companies seem to have opt-in policies, but more often than not, I find myself getting email from places that I never said "yes you can send me updates whenever you want"
The only acceptable automatic opt-in would be for something that makes sense. Something that may have a security or safety flaw or something. Otherwise, it should be a requirement that even "product update" emails are required to be opt-in.
I do not have a problem with companies having the checkbox checked for you by default on the signup form, but if you uncheck it, you can rest assured you will not receive any further emails from the company other than the relevant ones.