Why is obtaining a follow-up email from an event impervious to being spam? Attending an event is not consent for marketing. Marking spam as spam is not being an

Why is obtaining a follow-up email from an event impervious to being spam? Attending an event is not consent for marketing. Marking spam as spam is not being an asshole. I know it sucks when an IP gets burned, but when you're acting in good faith it's a rarity - or has been in my experience.