Well, that's debatable. Why are they trying to keep the legal advises they received about the validity of this marriage secret if there is nothing to hide?
I'm not a conspiracy nut but well there are dissident voices among constitutional experts.
It isn't Charles and Camilla who have ruled that the advice given to the Lord Chancellor be kept secret until after Charles dies but the Courts. It wasn't Charles that asked for it. It was a judge that denied a freedom of information request to see the advice given to the Lord Chancellor.
That advice will have included opinions that Charles could and couldn't marry in a registry office which we know as that point was made clear at the time - that there were arguments both ways. Based on the advice he received the Lord Chancellor ruled that it was legal for them to marry in a registry office. What the exact advice was - the wording of that advice and the arguments for and against in that advice isn't known and that is what is being kept secret.
The marriage is legal as there hasn't been a full blown challenge in a court but there were some papers lodged on the day arguing that it wasn't legal which were thrown out.
Do any of you really beleive that the Queen, Archbishop of Canterbury and PM would all attend the blessing knowing that the marriage wasn't legal? I don't and they would also have been privy to the advice given which formed the basis of the decision.
There were three Acts of Parliament that had to be considered - the first was the Act in the 19th C about marriage which said that royals had to marry in a Church. Then there was the 1949(?) Marriage Act that made registry office marriages legal but said something about the status of royals not changing but...in 2000 there is the Human Rights Act which overrules both of those by saying that everyone has the right to marry. Without the Human Rights Act the marriage would be definitely illegal but with the Human Rights Act the marriage in a registry office became legal.
In addition if the advice given was that Charles couldn't marry in a registry office then we know that he could go to Scotland and marry there in a church as Anne had done.
Charles marriage in the registry office also made it possible for later royals to do so as it set the precedent so if Charles' marriage is ruled illegal then Lord Nicholas Windsor's marriage wouldn't be legal and his descendents wouldn't be in the line of succession (although I am not convinced that the Vatican service isn't recognised in Britain anyway).
Constitutional experts always disagree on matters. The person receiving that advice from a range of experts then makes a ruling which is legal until successfully challenged in the courts where both sides would put their arguments and the courts would then rule whether the Lord Chancellor, in this case, or others in other cases, made the right decision. This is the same thing with many legal matters - differing opinions and advice given by different legal experts and the recipient makes their decision based on that advice and until a court makes a ruling that advice is the legal one.