פירוש על עבודה זרה 102:6

This is the end of the midrashic baraita. Leviticus 17:7 is the negative commandment against sacrificing on a private altar. There is a lighter penalty in the case where he dedicated before it was prohibited to sacrifice on private altars—it is only a negative commandment. This is unlike dedicating and sacrificing when private altars are prohibited which is punishable by karet, as the verse explicitly states.
In any case, this concludes the long difficulty. This verse is used here as the negative commandment for slaughtering on a private altar. So how can R. Elazar use it to prove that one who worships an idol in a way not usual to that idol is liable?