In other news, Blizzard pays less than $1000 to make sure modders can't make starcraft better than they can.
I am particularly confused by Blz's rationale. Valve basically own a significant fraction of its success to community: CS (based on modder produce) Dota2 (bought from a modder); these 2 are their big revenue sources. While Blz seems always intended to disallow any modder involvement at all. Most Tower Defense games own their popularity to the early days of SC and warcrfatIII. And Dota is basically just a warcraft III…
But what you need to understand about Blizzard is they are extremely conservative about the game experience. They're proud of their work and while they never consider it perfect, they want the one experience for all their players to be consistent and to have the consistent feel of Blizzard polish (which lacks with mods). They're the Apple of game design.
It's also a bit of a different question when you're talking about pre-wow and post-wow Blizzard. Pre-wow it's also a matter of "it's hard". Nowadays it's easier, and their philosophy is I believe slowly changing.