The market place. If everyone else was offering one-ways for 1300, you would probably see BA doing the same. It's a business, they will charge what the market will bear (for the product they offer).

BA offer one ways on euro routes, because it needs to compete against LCCs in a way most other European carriers don't. Although, I noticed AF offered cheap one-ways on many of their new flights from the regions (TLV-NCE etc). Although that whole experiment appears to have floundered, the approach appears to have been in response to LCCs building market share.

As others above pointed out, carriers have traditionally only offered deep discounts to those staying for fixed periods, typically a minimum Saturday night. One way pricing would frustrate this pricing strategy, which aims to charge business travellers more and sell cheaper tickets to leisure travellers, as these different markets can bear different prices and BA knows flights at certain time can't be filled by business passengers alone.

Such pricing strategies are not unique to travel. If you went into a supermarket and expected, to buy just one of a 2-4-1 (BOGOF) at half price, I'm sure you can guess the response.