FinCen has issued two basic administrative rulings over digital currencies;
The first one relates to the application of FinCEN regulations to a virtual currency trading platform, while the second discusses the application of FinCEN regulations to a virtual currency payment system.
In ruling FIN-2014-R011, FinCen states that any and all digital currency exchanges must become licensed as a money transmitter including:
“A person is an exchanger and a money transmitter if the person accepts convertible virtual currency from one person and transmits it to another person as part of the acceptance and transfer of currency, funds, or other value that substitutes for currency.”
On the exchange you are buying/selling bitcoins. The exchange carries the burden of giving you legal tender or a different cryptocurrency. According to the FinCen, regulations in the US, consider that the equivalent of real currency is treasury bonds, and to fully comply, the exchange carries the burden of maintaining their operations under the exact regulation.
FinCen confirms that all payment processors are obliged to register as money transmitters, comming to significantly increase the cost of entry in becoming a payment service provider, which could seriously affect the industry and the ecosystem.
FinCEN finds that any Company working with digital currencies would be a money transmitter pursuant to regulations and digital currencies become irrelevant to this fact:
“The Company is an exchanger under the Guidance because it engages as a business in accepting and converting the customer’s real currency into virtual currency for transmission to the merchant. The fact that the Company uses its cache of Bitcoin to pay the merchant is not relevant to whether it fits within the definition of money transmitter.”
To be able to fully operate as a money transmitter, a business wishing to continue operating must pay hundreds of thousands of dollars to become licensed in the USA.
In addition, companies are required to register with FinCen, conduct a comprehensive risk assessment of its exposure to money laundering, implement an Anti-Money Laundering Program based on such risk assessment, and comply with various record keeping, reporting and transaction monitoring obligations, in addition to meeting several other requirements. In other words they have to report the whole Company operation.
It is also interesting to note that the government classifies Bitcoin as a digital “property” for taxation purposes, but as digital “money” for regulatory and licensing purposes.
FinCen’s definition of money transmission and existing exemptions
A final type of convertible virtual currency activity involves a de-centralized convertible virtual currency that has no central repository and no single administrator, and that persons may obtain by their own computing or manufacturing effort.
A person that creates units of this convertible virtual currency and uses it to purchase real or virtual goods and services is a user of the convertible virtual currency and not subject to regulation as a money transmitter.
By contrast, a person that creates units of convertible virtual currency and sells those units to another person for real currency or its equivalent is engaged in transmission to another location and is a money transmitter. 
The Rule defines the term “money transmitter” to include a person that provides money transmission services, or any other person engaged in the transfer of funds. The term “money transmission services” means the acceptance of currency, funds, or other value that substitutes for currency from one person and the transmission of currency, funds, or other value that substitutes for currency to another location or person by any means.
The regulations also stipulate that whether a person is a money transmitter is a matter of facts and circumstances, and identifies circumstances under which a person’s activities would not make such person a money transmitter.
Also, FinCen regulations only take affect at certain dollar amounts; you can also try to get even further clarification from the Treasury yourself, because on this matter it doesn’t matter what kind of legal counsel you ask, nobody knows all the semantics until an issue comes up that is actually decided in the courts.
Link to source:1, 2
Photo source 1:igotbitcoin
Photo source 2:calvinayre
Photo source 3:acfcs