Grub is old enough to attend Junior High. Lilo would be in grad school. they both have windows compatability. Microsoft not reciprocating is evidence of their intent.
Nobody gives hell to every other OS manufacturer that will clobber our current bootloader configs. That means more than OSX, btw.
For better or worse, there is a difference between a system used by a tiny minority of people (again: on the desktop) and a system that practically has a monopoly on the (corporate) desktop and that is developed and sold by a company that has a history of anti-competitive behavior. (And to be fair, I get the impression that things have begun to change at Microsoft since Nadella became CEO. But that still does not erase history.)
Meanwhile, even that "good behaving" Linux distro will happily remove Windows's boot record in favor of GRUB's, rendering the system reliant on a component that breaks if you want to remove Linux (and most people are way more likely to want to remove Linux than to remove Windows!). Hardly neighborly behavior in and of itself. And yet, I'm still not seeing a reason to cast aspersions on GRUB or Linux distros, nor am I seeing those twirly mustachios on their faces.
No OS gives much of a damn about anything else running on the computer. Claims to the contrary are literally farcical and the pseudo-damages it could theoretically cause to the easily bruised are silly.
Most Linux distros (the ones I have used, at least) will detect an existing Windows installation and put a corresponding entry into GRUB's menu. Linux has been able to access data on Windows partitions for many, many years.
I agree that the boot loader issue probably has at most negligible impact on the adoption of Linux (or any other OS for that matter). I am just saying that in light of Microsoft's history one may feel inclined to judge them differently.