Joel, you aren't using the word "transmit" according to its legal definition. It doesn't mean the same thing as "trade". It means something like "trade via a third party." It is not the traders who are regulated. It is the third party "transmitter" who is regulated, for the benefit of the traders.
Sorry if I did not make this clear in my examples, but that's the reason I chose MtGox codes. In the case of MtGox codes, BitInstant acts as a third party "transmitter" between MtGox and the purchaser. In the case of Bitcoin, well, there is no third party since Bitcoin is just a communications protocol. The two parties, the buyer and seller, are simply engaging in trade, which the US at least has no jurisdiction to regulate.
Here is the full regulation (emphasis added):
Money transmitter:
(i) In general.
(A) A person that provides money transmission services. 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. “Any means” includes, but is not limited to, through a financial agency or institution; a Federal Reserve Bank or other facility of one or more Federal Reserve Banks, the Board of Governors of the Federal Reserve System, or both; an electronic funds transfer network; or an informal value transfer system; or
(B) Any other person engaged in the transfer of funds.
(ii) Facts and circumstances; Limitations. Whether a person is a money transmitter as described in this section is a matter of facts and circumstances. The term “money transmitter” shall not include a person that only:
(A) Provides the delivery, communication, or network access services used by a money transmitter to support money transmission services;
(B) Acts as a payment processor to facilitate the purchase of, or payment of a bill for, a good or service through a clearance and settlement system by agreement with the creditor or seller;
(C) Operates a clearance and settlement system or otherwise acts as an intermediary solely between BSA regulated institutions. This includes but is not limited to the Fedwire system, electronic funds transfer networks, certain registered clearing agencies regulated by the Securities and Exchange Commission (“SEC”), and derivatives clearing organizations, or other clearinghouse arrangements established by a financial agency or institution;
(D) Physically transports currency, other monetary instruments, other commercial paper, or other value that substitutes for currency as a person primarily engaged in such business, such as an armored car, from one person to the same person at another location or to an account belonging to the same person at a financial institution, provided that the person engaged in physical transportation has no more than a custodial interest in the currency, other monetary instruments, other commercial paper, or other value at any point during the transportation;
(E) Provides prepaid access; or
(F) Accepts and transmits funds only integral to the sale of goods or the provision of services, other than money transmission services, by the person who is accepting and transmitting the funds.
Read as you suggest, exceptions D and F would make no sense. A person who actually had more than a custodial interest in the currency wouldn't be a money transmitter, since they'd be moving their own money. Yet the exception specifically excepts only those who have a mere custodial interest -- that is, it excepts those who are moving other people's money but includes people who are moving their own money. And sale of goods or provision of services wouldn't be covered at all.
The regulations do cover people who take funds from one person and then deliver them to another person, even (in fact, especially) if the funds become theirs in the process. That's why it has to except the sale of goods and the provision of services.