Why is there so much baking soda in scones?
When I made scones for the first time, I also found the taste a bit unusual. The recipe I used specified a total of 11 g baking powder and baking soda per 250 g flour, and also 5 g salt. I used less salt, but the taste is definitely unusal for people accustomed to continental Kleingebäck. 

I can see two reasons for this. First, the dough for scones is quite unusual, something between cookie and pastry. It is a lot tougher than the usual soda-leavened dough. So I guess that it needs additional leavening power in order to create a light texture, unlike the semiliquid batters which raise without a problem. Plus, it gets no help from egg whites the way some cakes do (in fact, my recipe specified egg yolks only). 

The second point is one you already mentioned in the question. It seems that the English tradition is to eat scones with cream and jam. I tried it with my own scones, and the combination was quite good. I don't eat much jam, as it is too sweet for my taste, but the pairing with the bland scones was really nice. A sweeter or richer vehicle would have made the whole thing overwhelming.