I am genuinely intrigued as to why that's better?

Especially as in the second part of your comment you restrict shareability/usability significantly from what the author wants to achieve (IE I can't as a free user bash out a diagram and share it, then later do another and share with a different person).

In fact the solution is so elegant when you think on it; free account volume is incredibly scalable, zero storage cost, offer immediate value but also a really simple upsell to pro accounts (save to the DB so you don't have to save the URls).