I’ve always been surprised that governments don’t do anything against it. After all, currency is a Regalian power, and it’s forbidden to pay using other currencies than the country’s one in pretty much all countries.
The distinction of "legal tender" is that (1) the state expects payment in the domestic currency and (2) when there is a dispute in court, any fines or financial penalties will have to be paid in the domestic currency.
This does mean that if you enter a contract which states that some payment has to be made in a foreign currency, a court can overrule this and force the payment to be made in the domestic currency instead. That basically never happens in practice, but it is possible.
As to why governments don't forbid Bitcoin & friends: in the larger economy, they just don't matter, so why bother? Now the ICO craze has caught attention because of securities fraud, and governments have stepped in.
Going a bit off tangent, but I just want to clarify that the concern is primarily over token sales not meeting securities law requirements relating to registering a security before offering it to the public, and not securities fraud.
As it is, several token sales that have engaged in fraudulent practices have been pursued by the SEC and no one takes issue with this. Fraud laws are clearly applicable to all financial interactions, including those involving cryptocurrency, and their enforcement is a public good.
The controversy is in applying securities laws to token sales and thus placing the SEC as the centralized gatekeeper with which any party wanting to conduct a smart-contract based token sale needs to register with. Further requirements include meeting disclosure requirements designed for traditional securities which have questionable applicability to a project with no central coordinator that aims to create a protocol utility token that confers no legal claims to an underlying asset.