1) Dollars are considered legal tender by the U.S. government for all debts public and private.
Eh, point taken, but that is a phrase that occasionally does not mean what it appears to say to a layperson. For example, why can airlines require credit card payment and refuse my cash if I wanted to pay for their overpriced stuff in the air? Well, "all debts public and private" has a rather technical meaning....
It's not a debt until there is an irrevocable transfer of goods or services. It becomes a debt when that happens, until you provide compensation, and at that point, they must accept legal tender as payment. On the other hand, the "cash price" could be 10x or 100x the non-cash price, so it's ambiguous.
An example of a storefront which does not accept cash is "Split Bread", a sandwich shop located outside the front doors of the Metreon, in San Francisco: http://splitbread.com/locate/ Because you must pay before your order is activated, and because they do not accept cash, the consequence for being unable to pay other than cash is that is that the order is taken, but not activated. This is perfectly legal.
Another example of someone unwilling to accept cash or checks is the California Franchise Tax Board. They want a routing number into your bank account, nominally because the processing cost for them is cheaper than employing a person to run checks through a franking machine and type in numbers to achieve the same effect. You are heavily penalized for paying by check, and they will not accept cash payments directly unless you cog a specific field office; for example, neither the Oakland nor the Los Angeles field offices will accept cash. Effectively, this means that a cash or check payment must be made at the field office in Sacramento, San Diego, Santa Ana, or San Francisco.
The actual reason for this appears to be so that they can later reach into your account any time they want to and extract more money, without having to go through legal proceedings to get additional permission. For example, California bill SB 30 passed which institutes a retroactive income tax on California income earned from Jan 1 2012 or later, with the taxation being up to 9% of the earned income. Additionally, small businesses are being hit with a retroactive EDD tax for the last three years, and are in fact being assessed penalties on the amounts, since they didn't pay the retroactive tax that didn't exist at the time when it was retroactively due.