Amazon doesn't have a physical store front so they don't charge sales tax. Other states like you Texas and New York people might experience differently, but that's because sales tax is a State tax and determined by each State.
Brizzzle is right, the rates are the same. Sales tax is typically a percentage of the sale price. Around 7-8 percent is pretty typical. It also usually depends on what state, county, and town you live in. Each level of jurisdiction will charge a rate that combines for the actual sales tax rate. You can fin out the rates by looking at the tax tables for a sales tax return in your state.
The sales tax rate is typically determined at the place the transaction occurs. Meaning some people can save a few pennies by shopping in a different county. However, when it comes to Internet transactions, sales tax is usually collected only if the seller has a physical store located in the same state as the purchaser. I may be inaccurate about this, but I believe with Internet transactions the tax rate is then determined by the purchasers location, since the transaction typically occurs in the purchaser's property or home address.
Internet transactions are not typically exempt from sales tax, like you experience through Amazon, rather your State's tax return will often ask about significant Internet purchases in which sales tax was not collected; wanting you to pay sales tax through the income tax return.
As someone else said, earlier, sellers aren't charging tax or deciding the rate, they're just collecting it because they're required to by the state.