Facts give the answer to the question, much more than liar theories.
Prussia and the new 2nd Reich perfectly knew that Alsacians were french.
That's why they refused self-determination when they decided to take the territory for price of France's defeat.
Alsace was governed like an occupies territory.
All the deputies elected by alsacians were called "protesters" which was for them the only legal way to say that they kept on feeling french.
The claim of new Germany on Alsace was as serious as the claim of France on Rhineland in the name of roman Gaul having the Rhine as its frontier or as would have been a claim of Germany on the Netherlands or Belgium.
It was an old anachronic way of thinking territorial politics in an age that had become the age of nations and national identities.
You should try to refrain from insulting everyone by calling them "liars".
You say you present 'facts', and yet then just post a bunch of unsourced statements.
"Prussia and the new 2nd Reich perfectly knew that Alsacians were french." -- Where is the proof of this being a fact?
"That's why they refused self-determination when they decided to take the territory for price of France's defeat." -- Plebiscites for self-determination weren't exactly common at that point. So, why did France refuse a plebiscite for self-determination for Alsace in 1919?
"Alsace was governed like an occupies territory." - Granted. Much of the reason, though, was that as a Reichsland, fortifications and such could be built at will without the Federal government negotiating with the state.
"All the deputies elected by alsacians were called "protesters" which was for them the only legal way to say that they kept on feeling french." - As was pointed out earlier, many of these 'protesters' were in coalition against Prussian domination, and happen to be in coalition with pro-French parties. By 1914, they were no longer the majority.
"The claim of new Germany on Alsace was as serious as the claim of France on Rhineland in the name of roman Gaul having the Rhine as its frontier or as would have been a claim of Germany on the Netherlands or Belgium." - So, you're saying that Adolphe Thiers' comments should be have been a valid cassus bellum?