That's not fair. You have to assume that they have never heard the word spoken, or used it in conversation, otherwise they would know how to pronounce it correctly and do so (unless they are being willfully wrong, but I'll ignore that).
So, all people are doing is pouncing the word in the most intuitive way - if like="lyke", then Nike="nyke". This is common sense - what would be "stupid" is thinking "Now, there is nothing to suggest this, but I think that this Nike word must somehow be pronounced completely differently to every other word with the same structure, so I will make up a different pronunciation and hope I get it right".
They are not stupid, they just lack that particular piece of knowledge. A good shop assistant would graciously provide them with this piece of knowledge - perhaps when they ask to try on the "Nykes" you can say "Yeah, the Nikes, no problem" and help them out, make them feel comfortable and help introduce them in to this wonderful sport.