How can a positive post be considered as spam?
If it were not spam then your posts is probably "off topic" I've just translated the thread where you posted and I seem to agree that your post (containing content from the OP) is off topic.
The thread is self-moderated. Is this the reason for which thread creator can do whatever they want?
Actually yes the creator of the thread can do whatever they want like to delete post etc. but of course as long as it is inappropriate to the thread
The thread going to be a total trash because thread creator is deleting good post instead keeping them alive and deleting the spam.
No rule for this?
How can you assure that the one who deleted your post is the OP of that thread?