[Avodah] gzalas akum

[M Cohen]

RSZ writes ...this explains nearly all the differences in halacha between us
and them.
We owe them only the basic duty that every person owes every other:
not to harm them, not to steal from them, not to defraud them.  But we have
no positive obligation to help them in any way; we do have such a duty to
our brothers, precisely because they are our brothers.

This is true. 

however, given the above I have never understood hashkofically the rishonim
(& din) that gzalas akum is only asur m'drabbon.

why should gzalas akum (on a d'oraisa level) be mutar?
what about "..the basic duty that every person owes every other, not to harm
them, not to steal from them.."

Mordechai cohen