Why is Spotify charging sales tax while Google Play Music All Access Isn't?

I noticed that the gap in price has just gotten larger as I got an email from Spotify last week informing me that sales tax will be added from July onward, which is concerning because I actually prefer Spotify to All Access and want to keep using Spotify, but the prices are hard to ignore: With the 6% sales tax in my state of Pennsylvania, Spotify will now cost me $10.59 per month, while All Access will cost just $7.99. Since Google is clearly the larger company, why don't they charge tax in their service and why do these online services charge tax at all? As far as I know, Netflix doesn't charge sales tax and I'm sure that it will distress many if it does.