Is it normal for professors to give credit to their students even when they don't deserve it?
Probably the conclusion that you don't deserve credit is not totally justified. I am pretty sure that the final result contains at least some words from your drafts. On a more serious side, it may well be that professor could never have written the final result without having read your drafts. You know, the first sentence is the hardest, and professors also know how to procrastinate. But they usually often know ways around it! Having somebody else to start the writing process, and giving this guy a deadline (and you know he will make the deadline) and then starting from there is one great way to get started on time.

Actually, in previous days my professor asked me quite often to draft something for him (emails, abstracts, reports,…), but when he got my draft and started writing himself, most of my draft disappeared as well. In fact he even told me in advance, that this will happen, but having some draft helped him to get going.

Writing paper together with an advanced researcher often feels the same. I echo the other answers, that you can learn a lot from carefully studying the edits. And remember that writing the words in one thing, but getting results, sharpening formulations (not sentences) and structuring text is another thing. So just accept that he thinks, that you deserve first-authorship and be happy.