I think the problem here is...
He claims in a lot of his arrangements, that he is the arranger - which is completely bogus. What he's done is either directly transcribe something that's already on the piano (like his Comptine d'utre by Yann Tiersen)... which is wrong. He doesn't get the credit for that. Yann Tiersen wrote the piece and he has published books with his arrangements and transcriptions.
HOWEVER, some of his arrangements (like his Titanic theme) which again, has been copied from another version, has such poor notation that he hasn't even put the emphasis on the correct beat, or his rhythmic grouping is poor (as in the Skyfall by Adele).
However, since as far as I can see, he doesn't charge for his poor arrangements... the question is whether any major harm is done. Yes, there is probably some moral damage to those who did arrange the material originally for which he has just copied it... and that's wrong - but he hasn't really 'cashed in' on anyone elses success... has he?