>Why should there be a fundamental difference in a person's ability to modify one of the above, but not the other?
Because one could trivially modify the first, and people do all the time (and screw the law), whereas the second is, as you said, "next to impossible" to modify even if you want it.
>Just because the laws of Physics don't allow mere mortals to so (not at least without a highly sophisticated nano fabrication lab) doesn't seem like a very convincing principled answer
In what universe do "the laws of physics prevent this to be done easily without millions of dollars" is not a convincing answer?
Because one could trivially modify the first, and people do all the time (and screw the law), whereas the second is, as you said, "next to impossible" to modify even if you want it.
>Just because the laws of Physics don't allow mere mortals to so (not at least without a highly sophisticated nano fabrication lab) doesn't seem like a very convincing principled answer
In what universe do "the laws of physics prevent this to be done easily without millions of dollars" is not a convincing answer?