People are free to think and say both, but not free to act just as they may wish based on those beliefs.
You could never stop people thinking the McCanns know what happened to Madeleine. The problem is there are so many who do, and that's because this case is so unique in the UK. If it weren't not so many people would be so interested. But it is and there's nothing anyone can do about that.
I have no problem with what people think, unless they were so convinced that she is dead that they wouldn't even think of reporting a sighting (and yes, I'm aware of the tabloid excess that really hasn't helped).
ETA: And, yes, I do have a problem with people who attack this family, based on nothing other than opinion, in itself fuelled by others with an agenda.
But what about potential strange noises coming from a cellar somewhere? If you were convinced that she was dead, would you bother reporting it? Would you think that it is just a child playing and therefore of no importance?
That is how other children have been found.
Cops were SO close to finding two of the Dutroux victims alive... but they dismissed the cries that they heard as irrelevant. How sad is that?