All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
There is no contradiction between the fact that it is a Sunnah and not obligatory for a Muslim to initiate the Salaam and what was reported about the prohibition of abandoning him. To clarify this, a Muslim may not say the Salaam to his Muslim brother without abandoning him and without there being any cutting of relations between them or any dissension. So there is no correlation between the two at all.
It should be pointed out that a Muslim should not refrain from saying the Salaam to his Muslim brother if he meets him, as this makes him and the nation miss much goodness.
Abu Hurayrah, may Allah be pleased with him, narrated that the Prophet said, “You will not enter Paradise unless you believe, and you will not believe unless you love each other. Shall I tell you about something, if you do it you will love each other? Spread the Salaam amongst yourselves.” [Muslim]
The hadeeth which you mentioned in the question was reported by Abu Daawood and An-Nasaa’i, and Shaykh Al-Albaani classified it as hasan (good) in Saheeh At-Targheeb wat-Tarheeb.
Allah knows best.