My guess it's not really a technical reason but a cost one. The activation keys come from a 3rd party supplier and ED probably want to minimize the costs of having to re-authorize the BS1 -> BS2 upgrade keys for this beta bit. It's a case where they have to balance annoying some users (it is just beta access afterall) with the per-key cost during this pre-release period.

As BS1 was also available via Steam, and BS2 didn't make it there then it might be related to that as well i.e. a different pool of keys that has a different charge for them.

Like I said, just guessing but they probably don't encourage internet wrath for fun or at least no reason..