The Gemara interprets the word ‘Gadol’ in reference to ‘Onesh’ (greater liability) since laws of Shabbos apply also to things detached etc.; why is this related to as a ‘liability’ and not simply another detail present in Hilchos Shabbos and absent by Shemittah?

How do we understand the obligation (according to Rav and Shmuel) of a Korban Chatas for a ‘Tinok Shenishbah’ who had no prior knowledge or awareness of this mitzvah; why is he not regarded as a Ones?