you MAY NOT patch the binary
That depends. Does your software contains GPL'ed software?
Assume not, for simplicities sake.
In that case I don't know. What I do know is that XC16 and XC32 use the GCC compiler which uses the GPL v3.1 license.
And binaries made of GPL'ed software may be patched. Nobody can forbid that.
I guess you're right in principle. And the way GCC is built, there is now way Microchip could claim having isolated their contributions enough not to make the whole resulting compilers GPL. It's just impossible. So you should be able to do pretty much as you like with the compiler itself.
What they can protect though and not make GPL is all their support files for their microcontrollers (headers, libraries...) that they can't help but distribute freely. But those are the files that they should protect actually. It's just that it would be very unpractical to do so, so they protect the binaries instead, which is much easier (but not all that effective obviously).
On a legal point of view, I think you should be free to use XC16/32 as you see fit without any restriction, but I'm not sure you'd be authorized to use Microchip's support files. And without those, frankly, you wouldn't be able to do much. You could always rewrite them as long as it's obvious that they are substantially different from the original, but that sounds like a lot of work. (AFAIK, this is what the MinGW team has done for the Windows SDK headers and import libraries?. A lot of work.)
Anyway, according to the GPL, you should be free to redistribute the binaries themselves (even Microchip ones) as long as you distribute the source code along with them. Patching them is a different story - I don't know how that would be considered exactly on a legal standpoint. This would be a modification of the binaries that would make them not match the source code anymore, thus breaching the GPL somewhat. OTOH, circumventing artificial limitations in GPL software may not be considered as a modification at all. I guess the end of the story will only be known once a court settles a case like this.
I'd tend to doubt Microchip would take this to court though - they probably are borderline with the GPL so they may not want to risk losing a lot in court.