It's partly motivated by the standard "innocent until proven guilty" position that a Jury should always start with. The first consequence of this is that the Defence has to prove nothing -- and, indeed, need not offer a defence at all. The second is that, therefore, the Defendant can offer a defence that is entirely rejected, and that still isn't enough to establish guilt. As a result, it's not true (although it's common for acquitted people to claim otherwise) to say that the Jury accepted such-and-such a part of the Defence's argument. If the prosecution hasn't established enough evidence to be sure, beyond reasonable doubt, of guilt, then the Jury should acquit.
The other possibility, that I suggested earlier, is that the Jury agreed that the four were guilty of criminal damage, but decided to acquit anyway. This is their right, in part because jurors don't have to explain their verdicts anyway, and in part because it affords protections from the Law being an ass. This is often known as Jury Nullification, and has sometimes been useful to protect obvious miscarriages of justice (or, sadly, to perpetuate them); but, in either case, exists because the Jury has to be free to make its decision without fear of punishment. There'd be no point in having a trial by Jury at all if there were some objective standard by which to judge the correctness of their verdict.