If I recall Amazon recently lost their fight on this.... maybe why they are supporting the measure now. THEY want a "level playing field". and THEY have the money to lobby for this whereas small businesses don't.
They have changed their business plan where they expect to have a physical nexes in most states and would be subject to collecting ther sales tax anyway == I it is related to establishing very fast delivery. I think the fight they lost was in a State court and had to do with the fact that they used servers located in that state and the question was whether or not that established a nexes. Because of their change in business plan they did not appeal. Amazon has also found that they collect the sales taxes for many other business who where Amazon get the order and collects the money but the item is sold by the other business -- Amazon collects the sales tax for those businesses and charges them a fee for doing so. Wal-Mart also supports it I think, because they have a physical presence in all 50 states and already have to collect sales tax on all internet sales. Dell Computer probably by way of 'service' people also collects from all or nearly all the states that have sales tax.
I think if this is implemented, when the smoke clears, there will be very little new revenue to the states. Cars, boats, RV's, Mobile Homes and a lot of other big ticket items the sales tax is collected, not by the seller but when the item is registered. Many food items are exempt, clothing is exempt in several states, downloaded intangables (i.e. software) is exempt in many states, business to business transactions are usually exempt and any sales taxes due are usually paid by the business because they are usually already subject to sales tax audit and internet transactions will not be difficult to find, foreign sales, as much as 20% of transactions for lots of businesses are exempt. So that 225B number is in my opinion highly inflated since (I believe) it includes all of those things.