I doubt NetRegistry were sending you that web dev spam - there's really no need for them too - it happens _all the time_. The spammers are trawling the newly registered domain lists and looking up the domain contact details via whois. Doesn't matter _who_ you register domains with this happens (I've got several this week from a domain I registered using AWS Route53).

Net Registry are _very_ marketing pushy, so they've almost certainly recognised this and are trying to monetise it - but I strongly doubt they're going anywhere near "extortion". This is (in my opinion) less like "Nice place you've got here, it'd be a pity if something happened..." but closer to "Hey, we noticed you've moved into a kinda dodgy neighbourhood - have you considered one of our great insurance plans?"