Honestly it just seems like Starbucks has gotten lazier and lazier the past few years Christmas design wise. A plain red cup just seems like the entire design team just decided to phone it in so they could get out of work early.

You even see it in the details:
2010, detailed snowflakes of many shapes, not much empty background
2011, little details like the snowflakes seem to show less work
2012, just flat-colored shapes you can make easily on ms paint, no background
2013, not much to tell
2014, barely reminds of christmas
2015, it's so empty and simple some people don't see it's supposed to remind of christmas and get downright offended by it, believing it's something else

If you don't want people to interpret your art badly, then put as much effort and details into it so that everything is clear and no detail is left obscure.