It seems clear to me that there are two factors at play here.

Firstly, the programme is not a revenue-based programme but a margin-based programme. Therefore a GBP200 flight should not earn half the Tier points or miles of a prestige cabin GBP400 on the same run. In fact, given the massive margin on the Business/First products any TP awards for the bargain seats should be somewhere around 1-3 TP. Hardly worth it. That BA has such slim margin on it's economy products that it can't load in loyalty costs without making the product unprofitable or uncompetitive is another matter. There is an argument for bearing costs on poor margin products to gain market share or upsell other products; I would imagine the BA strategists are wondering about this point too.

The second factor is dilution of brand: the EC Brand. Even if a decision is made that they can afford to cut margin on the economy product by giving loyalty benefits, this will have an effect on the perceived value of the brand, and therefore the flight product to which it is tied, by the elite cabin customers. A move to Blue+ style tiering, or award of TPs to economy cabins disproportionate to the margin of the product, would devalue the product unless one performed a counter-balancing tightenting somewhere else. This would be by raising tier boundaries or reducing premium-cabin points. The resulting programme would be more activity based than margin based and would be weighted to trying to capture market share as opposed to maximising profit. Given the current position of BA and, say, BMI, it is clear why the former has chosen a margin based reward programme and the latter one that captures market share.