It's actually quite far from the truth. Tapered forks are similar in cost to (maybe slightly more costly than) straight forks, based on what I have seen with a large number of suppliers.
The retail price has nothing to do with the cost of manufacturering. If you can decrease your manufacturing costs, while maintaining the retail price, you've boosted your profit margin.
The ironic part is that many builders use straight 44mm head tubes and partially external headset to fit tapered forks exactly because straight titanium head tubes are MUCH cheaper to make.
Of course they are, but that has nothing to do with mould fabrication and carbon-layups for carbon forks.
That said, there is nothing wrong with a frame builder having to balance costs, but I find it slightly odd to blame the fork makers.
It's not a question of blaming anyone, rather providing an example of how the bike industry pushes new 'standards' that are driven by profit margins as opposed to genuine improvements in bike design.
Has anyone ever found an Enve 2.0 or 3T Funda straight-steerer fork soft and flexible?