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This is true to the point where you can barely believe services release the products they do. The experience on Popcorn Time vs HBO GO isn't even comparable. HBO GO serves their content through a laggy flash player that you can't even force to play HD. Why would anyone watch non-HD content in a terrible browser player that wastes additional battery when they can open Popcorn Time in the time that it takes for the HBO…
Are you using a computer? I think most people watch TV on a TV, where "legit" apps aren't based on Flash.
HBO Go is an online service for people with an HBO subscription through a Cable/Satellite provider that's billed as making HBO content available on other devices; while you might get some utility out of HBO GO on a TV (e.g., for streaming back catalog content that isn't available through HBO On Demand), its not really the primary use case.
OTOH, neither is using it through a browser; I think the mobile app (which does support HD) is the main vehicle. I'm somewhat surprised if the browser version doesn't, but not that surprised