Not sure why it can't be done, but the reason the higher power units don't do channels 8-14, is that they can't make them transmit at that low a power. The Wouxun KG-1000 is the unit I like, because it has the removable face plate and will do 50 watts. It say it will monitor 8-14 only because it can not transmit at that low of power.
The reason channels 8 to 14 are not included in mobile radios on GMRS is that FCC rules limit them to handhelds only. I suspect Midland thinks it would be more confusing to include them as receive-only or something like that.
§95.1763 GMRS channels.
The GMRS is allotted 30 channels—16 main channels and 14 interstitial channels. GMRS stations may transmit on any of the channels as indicated below.
(a) 462 MHz main channels. Only mobile, hand-held portable, repeater, base and fixed stations may transmit on these 8 channels. The channel center frequencies are: 462.5500, 462.5750, 462.6000, 462.6250, 462.6500, 462.6750, 462.7000, and 462.7250 MHz.
(b) 462 MHz interstitial channels. Only mobile, hand-held portable and base stations may transmit on these 7 channels. The channel center frequencies are: 462.5625, 462.5875, 462.6125, 462.6375, 462.6625, 462.6875, and 462.7125 MHz.
(c) 467 MHz main channels. Only mobile, hand-held portable, control and fixed stations may transmit on these 8 channels. Mobile, hand-held portable and control stations may transmit on these channels only when communicating through a repeater station or making brief test transmissions in accordance with §95.319(c). The channel center frequencies are: 467.5500, 467.5750, 467.6000, 467.6250, 467.6500, 467.6750, 467.7000, and 467.7250 MHz.
(d) 467 MHz interstitial channels. Only hand-held portable units may transmit on these 7 channels. The channel center frequencies are: 467.5675, 467.5875, 467.6125, 467.6375, 467.6625, 467.6875, and 467.7125 MHz.
§95.1767 GMRS transmitting power limits.
A point to make about power is that channels 1 through 14 are given as power limits ERP, meaning 0.5W is what can be radiated on channels 8 to 14 by the antenna using a 1/2λ dipole as a reference. Channels 1 to 7 are supposed to be 5 W ERP.
ERP is effective radiated power, meaning the equivalent transmitter power gain plus antenna gain minus losses to give an effective field strength. In acceptance testing they will usually measure this in a chamber so it's dependent on the antenna they actually use and in practice favors handhelds.
It's kind of difficult to do when you allow non-fixed antennas such is the case with GMRS. If you increase antenna gain beyond 2.15 dBi (that being what a 1/2λ dipole or 1/4λ monopole whip will have in the real world) you're supposed to reduce transmitter power proportionally. The OEM can't guarantee you actually use the exact same antenna configuration they did in testing.
In the case of the MXT275 and MXT400 the FCC test documented that the radio didn't produce in excess of 50W, which is all they needed to show to meet GMRS rules. It's ultimately a GMRS licensee's responsibility to select power and antennas and not exceed ERP on channels 1 to 7. The MXT400 showed 3.9 watts on low and the MXT275 5.9 watts on low in the FCC tests. To the letter of the rules mobile antennas probably should not have gain on channels 1 to 7 even at low power with these radios (which as Daniel points out they are locked into doing). The MXT275 is going to be close even factoring in lossy coax and connectors.
Only 15 to 22 simplex and the repeater input channels are specified as transmitter power limits (50 watts) with no ERP restriction. As long you don't deliver more than 50 watts you can use as much antenna gain as you want.
