I guess they registered the trademark, so that's all that matters.
"We never said they are healthy, we just let you know that the SLOGAN is ours and nobody else can use it!" would undoubtedly be their attorney's response if they were ever taken to task for false advertising.
I have no problem with companies selling rediculious stuff. But given today's typical warnings based on preventing product liability claims you'd think all footwear would come with a disclaimer advising that shoes have been proven to be detrimental to the wearer's health.