They probably use some widely used mailing software to manage sending e-mails to their mailing list/subscribers. Most of these software packages have a setting where you can give the recipient an option to unsubscribe.
Lots of companies that offer e-mail to the standard person (gmail, yahoo) might be more likely to put incoming e-mails without an unsubscribe option on their spam list. If Google puts your company e-mails in the spam box by default, it’s much worse (for the advertiser) than giving people the option to unsubscribe.
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