If you bought their weakaura and gave it out for free, it would be permitted.
It is against Blizzard's ToS to claim payment for addons - that is at least why Zygor had troubles at a point. Hence they are hidden behind, "subscribe to us" in this case.
Even more at so, if you did, just share it as you want. So what. It is an addon to a game they don't own.