Spam is unsolicited commercial email.
if I sign up on a site, click the 'yes email me' button, and then mark the email as spam when I get it, I am the one acting incorrectly, not the sender who is literally doing what I instructed them to do..
Unsubscribing from email lists can be cumbersome and non-uniform, which is usually why people just hit that spam button instead. It always provides them a uniform interface and largely achieves the same goal from their perspective.
People forget subscribing and hit "Spam", which has an impact on the sender reputation and affects future deliverability for that sender.
One of the factors I didn't mention in previous comment is presence of physical mail address.
Not sure what "unfortunately for me" is in reference to - BigMailer isn't a platform for cold emails (there are plenty others that specialize in cold email campaigns) and Amazon SES doesn't give a free pass on the practice either.