Does MN 60 contradict SN 12.15? If so, why is it so?
SN 12.15 says that "everything exists" and "everything doesn't exist" are opposite extremes and are both wrong view.

I guess there are different problems with each view: e.g. "everything exists" implies attachment and ignores conditioned/composite nature of everything; whereas "everything doesn't exist" implies nihilism and amorality.

I think that MN 60 concentrates its argument on one aspect, i.e. that the view that "nothing exists" leads to amorality:


  it can be expected that, shunning these three skillful activities — good bodily conduct, good verbal conduct, good mental conduct — they will adopt &amp; practice these three unskillful activities: bad bodily conduct, bad verbal conduct, bad mental conduct. Why is that? Because those venerable contemplatives &amp; brahmans do not see, in unskillful activities, the drawbacks, the degradation, and the defilement; nor in skillful activities the rewards of renunciation, resembling cleansing


For that reason, "everything exists" is the relatively skillful view.

Incidentally I think that people sometimes talk about a "two truths" doctrine, i.e. that "conventionally things exist but ultimately nothing exists". Another way to express the "conventional truth (or relative truth) versus ultimate truth" duality is to call conventional "skillful" or "skillful means".