Personally I think its the basis of long term thinking for any company (which is what the BBC is but with different shareholders) - gain market, sell your goods, make profit, support those dependent upon you whom you are dependent upon.
Funnily that could almost sound like a reason for DRM if you believed that DRM would help achieve selling goods - which is why of course companies use it regardless of whether the reasoning to use DRM or not is flawed.
Yes, the BBC uses a lot of open source and I'm sure would relish a world where it was easy to publish media without DRM, however, I don't see any reason, from the BBC heirarchies perspective to actually support the stance you are saying they should take when there is some risk for the BBC in doing it. Yes it might encourage some other companies to not use DRM, equally it might encourage other companies to be extremely restrictive in allowing the BBC to put their media online and isolate the BBC from its viewers (shareholders, license payers, supporters - call them what you will).
You are proposing an argument against the commonly held falacies that Flash and DRM can secure the media and allow it to be resold multiple times. I'm sure the BBC, who have made statements about open formats previously support this, however, making a stance that could damage the institution because the argument against DRM is not been made successfully to those using it is not a reason for the BBC to take the risk and annoy its viewers.