Yes, some are.
Totally bizarre claims about the law probably do belong on a skeptics site because skeptical research and skeptical patterns apply more than any specific legal knowledge or training:
- I read that some people oppose a yellow fringe on the US flag, when used in the courtroom. Does this signify anything sinister? Some people think it means their rights will be ignored. Is there any evidence that is true?
- This website says when a person receives legal papers with their name in ALL CAPITAL LETTERS, like JOHN DOE, that really isn't their name and they should not answer it. Is some sort of legal fiction involved? If John Doe answers the papers addressed to JOHN DOE, are they taking responsibility for this alleged JOHN DOE? Is there any evidence that John Doe != JOHN DOE before the law?
- This website about forfeiture says the US government sued a "misplaced" moon rock to get it back... instead of suing the owner. This sounds ridiculous and violates due process of the owner, doesn't it? Did the moon rock take the stand? Did it have counsel or represent itself? Is there a copy of this proceeding somewhere or is it a myth?
One of these is true, two are BS. None have been asked yet on SE... all are amenable to Skeptical research methods and levels of skill.
Things like "I heard you won't go to jail in California on your first shoplifting offence, is that really true?" probably should be migrated to a more professional site.
But Are there public laws dealing with extraterrestrial contact? was answered with reasonably authoritative references for that kind of subject matter. So it seems practicing Skeptics can do legal research for the sort of things that ought to show up here instead of a more professional-oriented site.
That is my sense of where a line might be drawn. Let's see if we can apply it to the examples of the OP
- If someone tells me they are going to commit a crime, and I do not report it to the police, have I committed a crime?
As is probably belong on Law and Legal questions. Depends on what the crime is, and phrasing to cite skeptical claims. If someone asked "On a TV drama they prosecuted J for murder because he knew K premeditated to murder L and J did not report K to the police. This fan's blog claims that is not realistic. Has anyone gone to prison for not reporting a murder attempt?" it probably belongs on Skeptics.
- Was the (US) Income Tax Amendment (16th) properly ratified?
Maybe belongs on Skeptics. Is there a highly improbably claim, e.g. "This website says noone has to pay taxes because the 16th amendment wasn't properly ratified? Is that true?" This might be the kind of question passed from Law and Legal questions to Skeptics. It is straightforward to research as other sites have done most of the work already and discerning the truth boils down to reading, evaluating sources, and logic. It could be migrated if Skeptics felt they couldn't answer, but I think it could be answered just fine.
- Is it illegal to take pictures of police headquarters?
Even if phrased as a claim for Skeptics, it is likely to devolve into a discussion between rights and security, and won't revolve around the likelihood of highly improbably claims. How rights and security currently get balanced in recent decisions is a better question for the experts at Law and Legal questions. It is the kind of question that would probably be migrated to Law and Legal questions. The answer to this question is probably "it depends" rather than "thats BS".