A great many people, Christians included, think that the threat of hell is intended to "inspire" obedience to God. So, of course, when one thinks that because of Christ's work on the cross, he is no longer liable to that punishment, there is no threat and, therefore, no reason to obey.

Actually, there is a better reason to obey.

[T]he pious mind...restrains itself from sinning, not out of dread of punishment alone; but, because it loves and reveres God as Father, it worships and adores him as Lord. Even if therewere no hell, it would still shudder at offending him alone. (Calvin, Institutes, Bk. 1, Ch. 2.2, Battles Tr. Emphases added.)