Pointing out correctly tested lines is unnecessary. They don't need attention. Placing markers on them creates visual noise that distracts from important information, like lines covered by failing tests.
By analogy, it would be odd for the C# compiler to underline every syntactically correct expressions in green.
That being said, I do find the ability to trigger covering tests via the markers' context menu very useful. Making this change would require moving that functionality somewhere else.
