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Companies are usually required to charge sales tax when they have nexus in the particular state. The definition of nexus (from the American Institute of Certified Public Accountants web site): \"Nexus describes the amount and degree of business activity that must be present before a state can tax an entity's income. If a taxpayer has nexus in a particular state, the taxpayer must pay and collect/remit taxes in that state. In general, nexus is created for income tax purposes if an entity derives income from sources within the state, owns or leases property in the state, employs personnel in the state in activities that exceed \"mere solicitation,\" or has capital or property in the state. The amount of activity or connection that is necessary to create nexus is defined by state statute or case law and/or regulation and, consequently, tends to vary from state to state. However, all states are limited by Constitutional principles, judicial doctrine and federal law.\"
The site you link to is great. It demonstrates that some companies are only charging sales tax where they're doing business or have fulfillment or warehousing facilities, while at the other end some are very cautious in expectation of a potential federal ruling or law on Internet sales.
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Note that even if you don't pay a sales tax when shopping online, you may not be out of the woods yet. It depends on local or state tax laws. For example, on the New Jersey state tax return, you have to declare a \"use tax\" for all purchases you made on which you didn't pay a sales tax or on which the sales tax is lower than the state sales tax rate.