Short story: Because your state says so.

Packlane is legally required to collect sales tax from a list of states under which Packlane has a "sales tax nexus." All collected sales tax will be remitted back to the state that requires the collection of this tax; no percentage of sales tax is kept by Packlane.

Another thing to remember is that sales tax is determined based on the state (and, in some cases, county or city) in each order's shipping destination address, not the state where your business is headquartered. If you are shipping your order to a state in which you are not Exempt from sales tax, your order will include sales tax at Checkout.