I suppose it's reasonable for a jury to decide that a prosecution should not have been brought, even if the criminal act can be shown to have occurred. I don't know what other body is in a position to make that determination, so I guess that in some kind of "natural law" view, returning an innocent verdict when the facts say guilty, is morally permissible.
But I'm not OK with juries deciding what the law is, or just completely ignoring the law. My experience (IANAL) is that jurors are interested in the judicial process, assess evidence carefully, and try hard to return a "correct" verdict.
I haven't served on a real jury; I did serve on a mock jury, in a mock trial for training advocates (barristers). It was a date-rape trial, and the "evidence" was equivocal. Interestingly, although all the jurors were sincere and honourable, the jury split male/female on the verdict (I'm a male, and I came down on the male side of the split).