That's strange. I've never been charged sales tax for app purchases on the Play Store. I'm in GA where we have 7% sales tax. I purchased Stars Wars Pinball game last week for $1.99 and there was no sales tax added.
most states you don't pay sales tax unless the seller has an actual physical presence in the state - whether that is offices or somethign like that.
that's why online retailers were having a blast on online orders and brick and mortars were pissed, and it's understandable why.
i remember when NY passed a law that makes sales tax applicable to online orders. states are also mad they don't get that cut now either.
in NJ i pay no sales tax on Amazon orders, but i do for google play orders. not sure what google has here in NJ. Amazon is building a warehouse in NJ so after that we will pay sales tax.
what is interesting is that if you look for jobs on Amazon, they have jobs listed in NJ - at some companies they seem to own like Audible, but yet no sales tax. one would think that could count as a physical presence but maybe they are not full owners or something. not sure of the details.