Of course, they are only legal tender for face value, and since Congress has the exclusive authority to "coin money and regulate the value thereof", these state legal tender laws are unconstitutional, to the extent that they assign a value to gold coins other than the value set by Congress.
But if you want to pay a $50 fine with $1500 worth of gold, you can do so right now anywhere in the US.
I think the main reason is for taxes...
“The actual laws being passed by these states are very simple. They recognize gold and silver as currency and therefore it is free from taxation. So when a citizen buys precious metals and over time they appreciate in value against the dollar, it is not viewed as a capital gain and is therefore not taxed.
The inflation tax on every person holding “fiat money” whittles away at his or her savings and the future purchasing power of their assets. While the dollar (The Federal Reserve Note) has lost more than 97% of its purchasing power since the Federal Reserve System was created in 1913, gold and silver have acted as a much better store of value, rising 60 and 20-fold, respectively, in their dollar-denominated price.
Devaluation of fiat money is no accident. Even John Maynard Keynes, the economist who developed today’s economic playbook for central planning through monetary policy, confessed: “By a continuing process of inflation, government can confiscate, secretly and unobserved, an important part of the wealth of their citizens.”
With an ever increasing number of states choosing to accept gold as legal tender they are signaling that the financial structure of the current system may be weakening and are preparing an alternative for their citizens if that structure falters or fails.”