Mastodon was actively pushing to be the default instance.
This criticism was co-joined with the criticism that the Mastodon organisation had set the mastodon.social as the default instance during the signup flow on the apps. While the frustrations are understandable and there are certainly valid criticisms on that decision, crypto spammers do not manually sign up via the official app, making it more of an airing of grievances than actual critique on the spam defense policy by mastodon.social.