IMO this strategy seems inspired by TikTok's approach for retaining new uploaders.
TikTok used to give new uploaders a visibility boost (i.e., an inflated number of likes and comments) on their first couple of uploads, to get them hooked on the the service.
In Anthropic/Claude's case, the strategy is (allegedly) to give new users access to the premium models on sign-up, and then increasingly cut the product with output from cheaper models.
Of course, your suggestion (better service for users who know how to speak Proper English) would be the cherry on top of this strategy.
From what I've seen on HackerNews, Anthropic is all-in on social media manipulation and social engineering, so I suspect that your assumption holds water.