With questions like these, you should first reverse them: why would Poland allow Germany to annex Danzig? What is the possible incentive?

Why would they just decide to give up their only access to the sea? What guarantees do they have that this will be the only piece of territory the Germans will demand, that the Silesian territories would not be next? Why would they possibly accept the word of Hitler, infamous liar, in any case? And how does a government that just hands over such a major piece of native territory expect to stay in power, especially knowing they have the backing of France and Great Britain, the winners of the First World War (ignoring for the moment the short-term weakness of that backing, which could not be known at that time)?

As for Poland's treatment of ethnic Germans in Polish territory, there was real discrimination, yes, though the Germans exagerated its severity. However, populations such as these were Trojan Horses for every early Nazi act of expansionism: it is likely that even if they were treated fine that they would be a stated reason to invade Poland in any case, because ethnic Germans (and the territories they inhabited) being separated from the Reich was considered intolerable by the Nazi leadership. Additionally, note that Germany claimed the Poles were part of an Anglo-French conspiracy to destroy Germany, and faked a border incident featuring attacking Poles to help strengthen their case for invasion, so the minority issue was not of sole importance in any case.