If I recall correctly, they claim it's perfectly within Blizzard's policies to give out the client (or "guide viewer") for free and then charge for downloading content (the actual guide material) for said client.
However, the way I see it, that is against point one in
Blizzard's UI Add-On Development Policy:
1) Add-ons must be free of charge.
All add-ons must be distributed free of charge. Developers may not create "premium" versions of add-ons with additional for-pay features, charge money to download an add-on, charge for services related to the add-on, or otherwise require some form of monetary compensation to download or access an add-on.
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Own emphasis.