You missed the GP's point: the problem is not that true negatives were found; the problem is that they were not published. Because they were not published, future researchers might waste more effort re-discovering them.
Indeed, though there is often a more insidious effect. Suppose there's a claim that treatment T is effective for medical condition C, but it's actually a "placebo effect". If one study showed a (perhaps small) effect, and other studies showing no effect aren't published, a lot of money can be made selling T to customers. If the true negative results are published, the makers (and prescribers) of T lose that income.
This is one of many reasons for the low level of publishing "negative" results. Another reason is linguistic: In English and most other human languages, it's hard to make a clear distinction between "Our study showed no effect" and "Our study didn't show any effect". Most listeners won't hear a difference between negating the object and negating the verb, and the media frequently reports the former as the latter. Managers of scientific organizations are as prone to this problem as the rest of us are. "We want studies that show results, not studies that don't."
And, of course, there's the general cultural resistance to "negativity". This easily explains why many pseudo-scientific beliefs persist centuries after they've been disproved.