From the microsoft license (not that of masm32)
You may copy and redistribute the Sample Code and/or Redistributable Code (collectively "REDISTRIBUTABLE COMPONENTS") as described above, provided that
(...)
(c) the REDISTRIBUTABLE COMPONENTS only operate in conjunction with Microsoft Windows or Windows NT;
From that, it means to be able to use microsoft assembler you must
1) make sure nothing that comes with masm is included in or with the final program.
Which is probably why masm32 says that you can not use it for non-microsoft systems. I can't assert that it does not include copyrighted material into the final binary.
Summarized, using masm is a legal hassle. Unless you know what you are doing, MASM + osdev is a tricky combination. To be on the safe side, one should not use it.
I traced a few of the other 'strange' clauses of masm32 to other clauses in the original document. i.e. PD and GPL code are at risk too. Wether that is because the original license indeed restricts that or wether they wanted to be on the safe side because the eula is horribly ambiguous, I can not tell.
Disclaimer: i'm not a lawyer. information at your own risk.