Usually when people say stupid or cruel things, they apologize and move on, instead of defiantly standing by their statements. But that doesn't happen in Trumpworld, and it certainly hasn't happened in the case of White House staffer Kelly Sadler's callous remark about John McCain.
The big picture: Once you’ve worked for Trump for a while you know that the worst thing you can do, the biggest show of weakness, is to apologize. He never does and never did (with one exception — the "Access Hollywood" tape). So staff knows that if they publicly apologize they’re actually MORE likely to incur Trump’s wrath than if they just move on.