What a damned good question. I don't feel that I can submit an very good answer, because my feelings are ambivalent.
On one hand, it doesn't seem to sit well with what I thought the ethos of the armed forces. I thought they had a similar kind of confidentiality culture as we do in the NHS. I thought that servicemen never divulged anything... any information had to come from the MoD.
I think the MoD is allowing his, purely to reinforce their credibility, as Wolfie said. The whole thing was a media pig-circus, and they came out with lots of egg on their face. I see it as a propaganda tactic that is in the same ball-park as those employed by the Iranian, when our sailors and marines were in their custody. Thankfully, this will not be traumatic for the personnel, as the experience they had in Iran.
Then, like Vultan said... why not let them make a bit of cash? I can only say that it goes against the grain, to my mind. If I was being utterly pragmatic, I would agree. However, it just doesn't sit well with what I perceived to be military culture... the reassurance that these people serve out of loyalty, and not for profit. That's what mercenaries do.
That said... in a society that pays for stories, and many are more expensive, trite and banal than the drama that these 15 people endured, should we not expect that they would have been made offers? I think that what it really boils down to is the shift in our society's values. Money is what drives our lives now. Everything has a price, but no longer has any value.
In the end, as reluctant as I am to condone it, I would rather see these people paid for their stories than some pseudo-reality-celebrity, WAG or politician-turned-convict. Until society gets its house in order, I don't have a problem with the servicemen taking the money and running.