At best guess, I'd say it's because they're not releasing
any info to the public. They haven't cleared anyone, they haven't released what they've found in any of their searches (either of MR's home, car, property, etc; or of the roadside searches that have taken place), and they haven't released any details of the interviews or polygraphs that both parents have likely taken. All that has been released is that there was a hit on the lake, and that may not have even been Dylan.
I believe that they're either playing their cards very close to their chest, or there's absolutely no information or evidence showing what happened to Dylan. Therefore, I don't really see this as a sign of guilt for anyone involved, it's just that nothing has been discovered so they have nothing to release.
I'm fairly sure that LE would know which vehicle Mark drove, and seeing as the vehicles have been released back to him (again, something that was not
explicitly released to the media, but was something that I believe was found true a few threads back), I don't believe they found anything of interest there.
JMO