I understand what you are saying OP and it's frustrating for you, but they have to draw the line somewhere. They can't just stop using the BMI scale without another tool to replace it with.
If they did what you are suggesting then, yes, although that body builder would get treatment, it would open the doors to a huge amount of ineffective treatment and wasting money, which realistically the NHS cannot cope with at the moment.
Fertility treatment is more effective at a normal weight, and pregnancy is also easier and smoother - it is just a fact.
I say this as someone who started treatment with a BMI of 33 and had to lose weight.