Assad was invited into Lebanon by the Christian president, Frangieh, who I think was afraid of the Palestinians and considered Assad to be a rational secularist. Also note that in the early war, the Shiites and Maronites were "allied" in an enemy-of-my-enemy sort of way, as both disliked the [mostly Sunni] Palestinians in the South of the country. (also fun fact: in the early stages of the war, Israel and Syria were bascially on the same side)
Though the war later became much more complicated in terms of factionalism, it's not inaccurate to consider the early stages to have been "Everyone against the PLO", with some but not all Sunnis supporting the Palestinians.
It's actually kind of interesting, because the Palestinians at the time had adopted some Baathist trappings, and while Assad was technically Baathist, he tended to steer towards a slightly different strain (more economically liberal, for one). Also, he, too, was not a huge fan of the PLO.
