I think it's more of a blending of what's racist vs what's classist in America.

The argument should be that regressive taxes like soda taxes, much like speeding tickets, affects the poor at a much greater clip. Who cares about a soda tax or a speeding ticket when you make 500k a year? If you make that much and you want soda or you want to speed, you're going to do it. However, if you make 28k a year, the calculation is much different.

In America, poverty and race are strongly correlated (and there's causation, but I digress). Because of that, it's easy, although inaccurate, to use classism and racism interchangeably.

Calling someone racist creates a larger emotional response and is more damaging to social equity than calling someone classist.

Hence calling soda taxes racist.