Im not a law expert so grant me that. But in most developed countries, sending anonymous payments to anonymous users, tax free, and/or using an alternate currency within those borders is absolutely a violation of law. It is in the US for sure.
FinCEN just provided guidance stating that Bitcoins can be used to pay for purchases from merchants.
This is irregardless of the merchant remaining anonymous and/or the customer operating anonymously.
So, anonymous payments to anonymous users is most certainly not something FinCEN has indicated that it has any problem with. Now, of course, if that transaction incurs a tax obligation, simply because it was paid using Bitcoins does not remove any obligations, so taxes would still be do. But that doesn't make using Bitcoin illegal.
Care to clarify your statement?
If you're not paying taxes on those transactions, it's illegal. That's cut and dry. By US policy anyway. I will confine my statements to the US for now. Again, I think there is too much grey area and that's exactly why they are moving on this online tax bill. Which only hits major retailers now but is just the platform to go after everyone else.