I don't know what people are expecting the inside of the store to look like, but if you wanted a replica of a 19th century 'main street' candy store, you'd have to visit a museum, not a Disney park.
Take, e.g., the Christmas store in Liberty Square. It is an impossibility since there were no Christmas stores in Colonial times and 99% of the merch in that store is from the modern era. But... it has a Colonial facade!
How about the giant mall on the other side of Main Street with Mickey shirts and other modern merch displayed and decorated like it was a modern store? Where are the complaints that it doesn't look like a ye olde general store on the inside?
How do you have a 19th century candy store interior design when you're selling modern confectionery? Should you be seeing Skittle bags and Milky Way bars in wooden barrels and wicker baskets?
At some point in a theme park you suspend your belief and disbelief and deal with modernity being wedged into a historical setting.
I'm not saying the store should look like it fits in Tomorrowland, but, just enough nods to Main Street in the furniture, architecture, and decorations is plenty for what it is.