Adam Clarke on Joshua 22:19

If the land of your possessions be unclean - The generous mind of Phinehas led him to form this excuse for them. If ye suppose that this land is impure, as not having been originally included in the covenant, and ye think that ye cannot expect the blessing of God unless ye have an altar, sacrifices, etc., then pass ye over unto the land of the possession of the Lord, wherein the Lord's tabernacle dwelleth, the only legitimate place where sacrifices and offerings can be made. We will divide this land with you, and rather straiten ourselves than that you should conceive yourselves to be under any necessity of erecting a new altar besides the altar of the Lord our God.