Current LH policy on tax refund for nonrefundable fares seems to be the following:

If you have not used your ticket at all, you get all taxes refunded w/o a problem.

However, if you have used only a portion of your ticket, they will recalculate the price accordingly (LH terms and conditions). As the recalculated price is almost always (much) higher than what you originally paid, LH feels free, to keep the unused taxes to cover part of the price difference and tell you to be happy, that they don't ask you to pay the rest.

Whether this practice is legal or not is an open question, but who will go to court about the refund of taxes?