They are not taxes. They are pretty much what each airline is charging for the 'free flight'. Unfortunately the big groups IAG,LH and possibly AF/KLM are overcharging for redemptions. Ex US is one of the poorest way to use your avios. I will give you an example for a random day i picked up.
09/03/2020. SFO-LON. BA is asking £500+ plus the avios.
LH is asking around 600+ euros plus the miles.
One the other hand if i book with TAP via Lisbon i will only pay 26 euros or with United via Frankfurt 82 euros.
The above prices are for C.