It's here because someone found it worth sharing. From the HN guidelines:

    Please don't submit comments complaining that a submission is
    inappropriate for the site. If you think something is spam or offtopic,
    flag it by going to its page and clicking on the "flag" link. (Not all
    users will see this; there is a karma threshold.) If you flag something,
    please don't also comment that you did.

http://ycombinator.com/newsguidelines.html

Einstein thought in musical ideas.

From the down-voting count, it looks as if it's your comment that doesn't belong in HN, rather than the musical post.

(And maybe my comment too, but mine is double-meta, so I would understand any down-voting to it).