Yeah, Ghost is a option to disable in full version I think but overall it has been a part of official tetris rules for long while now, and it's default inclusion is in line with official Tetris guideline afaik. Overall it makes fast placement easier because in good play you are little more focused on bottom of the screen than the actual active block.
Checked in Wii U version. An option to disable Ghost is indeed there, in separate options menu for the whole game that is not in the Demo at all. This is a universal setting though so it disables ghost for all modes of Puyo Puyo and Tetris or enables it universally, so there is no disabling it for tetris but not puyo puyo etc.
I think it is a good feature especially in semi higher level of tetris etc. but yeah. Full version should have option to (universally) disable it.