It only causes a negative feel because of the fact that they are not being regulated, its market value can be manipulated by those people who are in capable to transact at a higher volume. Also, another reason why the government warns people to take precautions is because of Bitcoin being digital, which is a clear sign that it may only be temporary as no physical sign to support that it will not fluctuate gradually, and the fact that it can be a tool for money laundering schemes.