I was going to say "Nobody would know how to do this" but in reality, it's probably a bit more like "Not enough people would know how to do this" and if they're taking increments of 0.2BTC, they're hoping or expecting to get some real volume.

Making it just hard enough that anyone with a Coinbase account and a credit card can't just do it without actually running a full node of their own, would probably be bad for business.