Reasoning for overdubbing British shows to American?

I have seen this a couple of times on TLC for those sensationalistic accident and tragedy ‘documentaries’ as well as Teletubbies on PBS (how’s this for a divergent range of interests?). The narrators voice has been overdubbed from the original English accent to an American accent.

What would be the purpose of overdubbing the narration?

I assume that the original accent would be a BBC news sort of accent rather than an obscure or heavy northern, or Welsh accent that Americans may have difficulty understanding. The participants of the shows are not dubbed, and are usually understandable (although there are times when you would have to listen attentively).

With regards to Teletubbies on PBS, I suspect that there may be concern that kids would not pick up what the narrator is saying, but would the same not be true for those kids who are growing up in an environment with a southern drawl as the predominant way of speaking when they hear a ‘network news’ accent?

I think you’re right in the case of children’s shows, any American accent is probably seen as being easier for a toddler to understand than a British accent. However, nobody seemed to have any problem with Ringo as the original conductor/narrator on Shining Time Station.

Since dubbing is cheap, might as well appeal to the target audience by using a native accent with native lingo. I know the accident “documentaries” you mentioned. Not everyone on this side of the pond knows that Brits refer to a vehicle’s luggage compartment as a ‘boot’ or a truck as a ‘lorry’. Finding out that the criminals are fleeing over the theft of a few quid is downright puzzling to the uninitiated. Perfectly acceptable in Britain, confusing in America. Hence, dubbing.

Who knows why… but sometimes it really takes away from the show…
I remember a few years ago I was in the UK and I saw an episode of a fairly new show called The Crocodile Hunter. Of course, that crazy Aussie giving his narration is half of what makes the show entertaining… not long after I got back home, the local Discovery channel showed the very same episode I’d seen in the UK… but dubbed out his voice with a boring American/Canadian one! I emailed the station to complain… they said they were not aware of the change but would look into it… soon after the show was being narrated by the regular guy… Hmm…