I agree with the question. If reduced thrust is not approved due to unpredictable effects of the contaminant, and thus the regulator says - economies aside, full thrust please! ..... Derates being allowed simultaneously: it does not seem to follow the same logic.
The fact that you could lift even more with DRT compared to TOGA scenario (which is well understood and accepted) feels ironic as we get:
- a heavier aircraft and
- less thrust at the same time,
rolling over a surface with characteristics so suspect the regulations demand TOGA in the AST/FLEX case. Uneven judgement?