"Only" qualifies "need". "you only need to wear a seat belt on public roads" and "you should wear a seat belt on private roads" are not contradictions.

You're confusing a declarative v. an imperative statement. What you propose is a declarative, and you're correct that it ain't contradictory.

The WHO's messaging, however, is imperative¹ (emphasis theirs):

"For healthy people wear a mask only if you are taking care of a person with suspected 2019-nCov infection"

This makes it clear that it directly contradicts the more widely-accepted similarly-imperative advice of "wear a mask in public, no matter what".

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¹: https://www.who.int/images/default-source/health-topics/coro...

In English, "you only need to" is completely different from "you need to only".