Although it bucks the tradition of using LSB, the use of USB allows a simple AM receiver with a BFO (beat frequency oscillator) to detect and recover usable audio. LSB appears "inverted" to an ordinary AM receiver. I suspect there is a population of "legacy" receivers that the policy is intended to "include".
A short technical comment follows for others on the thread that might be interested in how and why the signals are constructed in a particular fashion.
USB and LSB are forms of "single-sideband, suppressed carrier". The carrier and modulating audio are mixed on a balanced bridge. The output is "double-sideband, suppressed carrier". Ordinary AM is "double-sideband, full carrier". A narrow filter tuned above the carrier frequency permits USB to pass and excludes the LSB signal. A similar filter tuned below the carrier frequency would pass LSB and exclude the USB signal. Since only the sideband carries useful information, it is advantageous to strip out the carrier and the redundant side-band.
For those of you who are familiar with CB but not amateur radio, you might have heard single sideband on the CB channels. When you listen to a single sideband transmission on an AM reciever (which is the only kind most CB transceivers have), the voice sounds like one of the adults from a Charlie Brown cartoon. You know what I'm talking about, right? :)