So turn off class coloring for the Unit Health status, set the Unit Health status to use a white color, and/or change the Bar Texture option? Not sure why you're complaining about these things when there are plainly marked settings available to change them.
It's called a question, not a complaint. That's probably the reason, I ended the sentence with a question mark...
If by "for a reason" you mean "slavishly copying whichever addon did it first", then sure... personally I find green heal bars unsightly, and Grid's shading approach much more intuitive. That said, there is already a ticket requesting options for the healing bar color, and I've already marked the ticket as accepted. If you want it to get done faster, feel free to write the code yourself and post a patch.
Thanks for your unconstructive and idiotic reply, feeling better now ? :)