International law is clear on the definition of refugees. The 1951 Refugee Convention didn't give Palestinians a pass where their refugee status can be considered different from all others; it said that Palestinians who were getting support from UNRWA were excluded from being protected under the Convention as long as UNRWA exists.
Which is why UNRWA exists 67 years later.
But there is only one definition for refugee.
And indeed, the UNHCR lists five countries that are the source of the highest number of refugees - Syria, Afghanistan, South Sudan, Myanmar and Somalia - of which only Syria has created more refugees that UNRWA has. Yet it doesn't include "Palestine" on that list.
One gets the impression that UNHCR is almost embarrassed to be sometimes forced to include Palestinians in its count of refugees because it is dealing with people who are in real crisis, not people who live as citizens in their own lands or who have been excluded by law from becoming citizens in the countries in which they have been born, which other international conventions call for.
In short, there is no international law that singles out Palestinians to be treated differently from other people; that idea is anathema to international law. The claims that Palestinian "refugees" are refugees under international law are ludicrous.
(full article online)
PA claims Palestinians remaining "refugees" forever is "international law" ~ Elder Of Ziyon - Israel News
The reason why people left is irrelevant to their right to return.
Actually, the alleged "right of return" is specifically the issue in question as it applies to some claim islamics want to apply to generations of Arabs-Moslems.