In that, I focus on what you might call ethics. But as for the legality aspect:

Through most of the civilised world (at a minimum, those that I know of definitely: AU, NZ, all EU, UK, CA), sending unsolicited commercial messages at all is illegal, with very little in the way of exception or subtlety. The USA’s CAN-SPAM Act is the outlier, unusually permissive, by being largely opt-out rather than opt-in like everyone else. (And frankly it’s not at all uncommon for the USA to be an outlier in a bad way among first-world countries.)

And taking the legality aspects further, it’s common for people to ignore restrictive laws that aren’t being particularly seriously enforced, and spam legislation is often like that around the globe. Just because lots of people are doing it doesn’t even mean it’s legal.