That's handy, having the slate wiped clean as to put it, .. seems a bit one sided and unfair to me. or am I missing something?
I'm glad you asked, because it's quite an important question.
Basically, the Allies wanted to justify their actions post-war in Germany. They couldn't do things (like the Nuremburg Trials) legally if they were just an occupying power according to the Hague Conventions and other agreements, so they considered that the German Reich had ceased to exist and that sovereignty was in the hands of the occupying powers. That's what allowed them to legally justify the border changes and mass population transfers, because it was done under Allied jurisdiction. In essence, if the German Reich had continued, then the Allies could have been legally liable for various acts. Also, by claiming sovereignty, the Allies could make sure that they controlled the restoration of the territory.
They also used their position to claim German assets abroad, for instance. It also means that Germany has no claim against the Allies for what they did (for instance, the destruction of Dresden) because there is no legal continuity. They were also effectively forced into accepting that the Federal Republic 'is the land of all the Germans' and that there are no more territories housing German people - so they can never make a claim against Polish territory for instance.
It's not the only time that this has happened - the 1992 Federal Republic of Yugoslavia was denied the right to be the successor state to the Socialist Federal Republic of Yugoslavia, for instance. From what I remember, quite a bit of SFRY property located outside the former Yugoslavia (especially embassies) was lying empty for years until they agreed how to divide it between themselves.
So, you can say that the slate was wiped clean, but they also lost the ability to make claims of their own, and they had to accept their country being shaped by the Allies rather than by the Germans themselves.
So, any territorial claims of today German state are invalid and null by default.
Correct. There's no legal basis for Germany to make any territorial claims. If anyone in Germany tries to make such a claim, they can be laughed at. It's the same reason why Germans can't try and claim properties here.