In the land promised them they will be tempted to substitute Canaanite deities for Yahweh and to swear by them. Oaths formed part of the social and commercial fabric of the time, and they are not forbidden here. Contrast Christ's teaching (Matthew 6:34 f.). In trading with Caananites it would require courage to refuse to swear by their gods.
Deuteronomy 6:10-15
10 And it shall be, when the LORD thy God shall have brought thee into the land which he sware unto thy fathers, to Abraham, to Isaac, and to Jacob, to give thee great and goodly cities, which thou buildedst not,
11 And houses full of all good things, which thou filledst not, and wells digged, which thou diggedst not, vineyards and olive trees, which thou plantedst not; when thou shalt have eaten and be full;
12 Then beware lest thou forget the LORD, which brought thee forth out of the land of Egypt, from the house of bondage.b
13 Thou shalt fear the LORD thy God, and serve him, and shalt swear by his name.
14 Ye shall not go after other gods, of the gods of the people which are round about you;
15 (For the LORD thy God is a jealous God among you) lest the anger of the LORD thy God be kindled against thee, and destroy thee from off the face of the earth.