Much as I hate to say it, I think this is actually quite a clever ruse.
JM hopes JKR will sue. Because then he gets A LOT of publicity and money from a 'JKR is suing me please donate' funder.
And the stickers are priced not to sell, so he can say that no-one bought them and so there is no detriment.
The other option is of course that he hopes the stickers will sell, and he genuinely thinks there is no problem, copyright or otherwise, with the slogan. But that seems.....unlikely?