"Without concession by" does not mean "according to".
The most likely interpretation (as pointed out by FumbleFingers in comments) of the (ridiculously long and complex) sentence is that:
Exploitation (...) should be penalized-without-concession by the national legislation.
Here we see that "without concession by" is not actually a phrase. "By" is a preposition connecting the verb-phrase "penalized-without-concession" to it's subject, the "national legislation" (an awful noun meaning in this context the body that enacts penalisation, rather than the written laws that "legislation" would normally mean).
And the phrase "without concession" means "without reducing the penalisation for any reason - without conceeding that there might be a reason to reduce it"
Another different interpretation of the sentence (my initial interpretation) is that the sentence was trying to sat that the "State concerned" should prosecute and penalize exploitation even if the national legislation in the visited country allows exploitation, and hence would allow concession. The States concerned should not "make concessions to" the national legislation, nor allow concessions by the national legislature, they should stick to their own position and prosecute to the full force being proposed. In this interpretation, the State is doing the penalisation, and the direction of the statement is that the State should not allow concession by the national legislation.
Both interpretations are similar in that the phrase "without concession" is telling us that no matter who is doing the penalization, it should be to the full extent possible.