I am a new GMRS user and no HAM experience and got am H3. I found this out when I tried to add some Valid GMRS channels higher up in the channel number ranges where it won't accept them. It also seems to restrict simplex channel entries above a certain channel number. Since I am a noob I can fit all the channels I want into the restrictions they have but yes, I find it weird that the radio does not just simply enforce the GMRS rules but let you place duplicates (maybe with different codes) in any channel slot you want without restriction.
Maybe, when I get more into radio, if I get a HAM license, I will just unlock it to "normal mode" to give myself more flexibility. But yes, even a noobie GMRS user like myself noticed this ridiculous limitation. At first I just thought I was not understanding it, but No it is a restriction of the way the firmware/software on the radio works.