This has been eating away at me as well. It's not that I feel it was inappropriate that they forgive him, I get that. They are his parents, and of course they love him and want to forgive him. I am of the mindset that forgiveness most benefits the person forgiving, rather than person being forgiven, and if forgiving him brings them some comfort I am glad of that.
However.
One does not need to verbalize their forgiveness to another, when it is for themselves. And it is very clear that their need was for CW to know that he was forgiven. And in this context yes, especially given who was in attendance, it felt very inappropriate.
I think that, whether they came to it themselves, or they had help, that the Wattses have come to realize that this may very well be the last time they ever speak to their son. It has become painfully obvious over the past couple of weeks that he does not wish to speak to them. I think this was their way of saying goodbye, in a sense, this was what they wanted him to know, in the event that they never talk to him again.