I think a partial answer to the question (why is it OK?) is that boomers are in a similar category to white people, i.e. privileged, so boomer bashing is more like "fuck off whites" than "fuck off blacks".
As a boomer myself, I don't mind the brickbats. On a conceptual level (for want of a better word) I am an unwavering devotee to the boomer creations of 1967 and 1977 et. al., but I think boomerdom generally bears a greater responsibilty for the betrayal of those ideas.