It may very well be legal tender. However, it is not intended to be used as currency.
The US government has the constitutional authority to mint coins and regulate currency, and they have done so.
You can argue that our paper money system is unconstitutional because the constitution specifically says "to coin money".
However, I have a hard time understanding how people who claim to be constitutionalists suggest that the government can't stop people from overtly trying to undermine the value of their currency by introducing their own and suggesting it be used in place of the dollar.
Actually, it's unconstitutional because no state shall make any thing other than gold or silver coin tender in payment of debts, but that's neither here nor there.
Alternative currencies are common across the nation. I posted a few examples earlier in this thread - Ithaca Hours, Bay City Downtown Dollars, and my nearby city of Nashua has its own Downtown Dollars.
The idea is to keep value circulating in the community, instead of being hoovered up into international banks and stripped out of the community.
If the feds are taking the position that it's illegal to barter if shiny bits of gold or silver are involved, then they've bitten off quite a chunk for themselves.
Recently they tried to prosecute someone for tax evasion for paying their employees in the uncirculated coinage based on the government-approved face value. They can't seem to make up their minds, though I guess it's understandable because it's the government.