Yep manufacturers have not supported 'home made programmers' for some time now and will not release the programming algo for the very reason you stated and the fact that its another source of income for them

My info is extremely diffrent. I asked my source at a given manufacturer why and were told that too many claims of insufficient flash retention were due to insufficient programming "force".
Since the complainers all claimed that the programming "force" used was sufficient and to spec and thus kept blaming the manufacturer, the manufacturer decided to only release programming data to those that would submit their programmers to the chip manufacturerer for verification.

Also, I know of no manufacturer that sell programmers.

Erik

PS in the above "force" can be pulse height, pulse width, Vpp voltage or whatever.