Boring but sensible answer to lots of versions of the same basic question (ISP/telephone line quotes, retail prices, etc):

The US is made up of tens of thousands of mini-jurisdictions, each with their own unique rules and regulations. Sometimes taxes, permits, fees, etc change from place to place and the price is going to be different from one place to another. When you have a national-level marketing department, it's easier just to quote the price you know applies everywhere, and let the locals add the rest on top.

For phone service it's really crazy. I used to work for a phone company, and there were outlier counties that charged $12/month in fees (50 cents is closer to normal) to each phone line for access to 911 service on what was nominally a $15 landline service. We couldn't just honor the $15 price and eat that $12 on top of the rest of the local fees and taxes - we'd have been losing money with each additional customer.

So the result was the national advertised price was still $15, and depending on your jurisdiction your out-the-door cost was going to be between $20 and $40.

And there's nothing indicating how much that tax is. You either know or you don't.

because sales tax!