Maybe Blizzard is paying patent trolls or otherwise buying patent protection for something stupid like XML parsing that should never have been awarded a patent; a lot of companies do, since it's cheaper than fighting. But there are
tons of library options to do XML parsing and image handling that don't involve (L)GPLv3. Can't imagine why they WOULD use it.
If they are using LGPLv3 libraries, then presumably their lawyers are OK with it, or they failed to run it past legal. Regardless, it's completely irrelevant to my point what Blizzard uses or doesn't use.
If you want to see a lossless compression format on the Web, you need the format to be picked up by, at a minimum, Google and Microsoft, the owners of the two top browsers in the world today.
Firefox would also be important, but would certainly follow if Google and Microsoft stepped up to support it. So really it only matters what Google and Microsoft think. If it's a license that they can freely use, then there's a chance it becomes a new Web standard. If it's only available LGPLv3, then Microsoft and Google would need to buy LGPLv3 exceptions in order to use it. A much bigger barrier to entry, and at least Google might object based on the concept that Web standards should be open. Mozilla would certainly resist if they weren't open -- though they eventually caved on H.264.
If you want to see adoption where it counts, you need cooperation from the companies behind the browsers with the market share.
No one is expecting Starcraft 2 to be the next big Web plug-in, so it's irrelevant to the product that they have LGPLv3 code in it (if they do). And if they do, Blizzard better hope that their lawyers are right in their interpretation of GPLv3, and that no one sues them hoping to take home a share of the profits you've pointed out that they are rolling in by forcing them to buy a license to get around the GPLv3 restrictions that they may or may not be violating. That's an expensive lawsuit even if they win.