I think this is a marketing trend first noted here: 

http://techcrunch.com/2012/04/01/bootstrapped-startup-saves-over-100k-by-dropping-ie/

Startups who are appealing to a niche that doesn't use IE. Given the odd attitudes of Silicon Valley types it almost makes sense. I don't think their goal is to serve the largest number of customers -- it's to get bought. This is one (odd) way to get attention of a certain type of user/investor/buyer.