There's plenty of smaller sellers that would love to be in compliance, but due to the nature of US sales tax and how FBA works (i.e., you don't know where all your product has been stored) it's practically impossible for them to be in full compliance, so they don't even try. Sometimes better to stay off another state's radar completely than get drug into a spat over partial compliance.
Say you bring a new widget to market, as a small business, and you start selling via FBA. Pick your favorite kickstarter project and imagine it's you.
There are thousands of sales tax jurisdictions in the US, between states, counties, and cities. There is no way you, as a little guy, can manage all that.
Right, and how are you proposing to enforce that legislation? Passing laws solves nothing by itself. How would you get buyers to file whatever you ask them?