I also thought it was interesting that Joshua said to give God glory, and make confession unto him; AND tell me what you did, but from what I see, Achan only confessed to Joshua, I'm not even sure he confessed to God, or repented... but as stated earlier we are not to judge Gods judgment, we can only read what we see
Joshua 7:19-20 And Joshua said unto Achan, My son, give, I pray thee, glory to the LORD God of Israel, and make confession unto him; and tell me now what thou hast done; hide it not from me. And Achan answered Joshua, and said, Indeed I have sinned against the LORD God of Israel, and thus and thus have I done: When I saw among the spoils a goodly Babylonish garment, and two hundred shekels of silver, and a wedge of gold of fifty shekels weight, then I coveted them, and took them; and, behold, they are hid in the earth in the midst of my tent, and the silver under it.
Joshua 6:18 And ye, in any wise keep yourselves from the accursed thing, lest ye make yourselves accursed, when ye take of the accursed thing, and make the camp of Israel a curse, and trouble it.
Joshua 6:24 And they burnt the city with fire, and all that was therein: only the silver, and the gold, and the vessels of brass and of iron, they put into the treasury of the house of the LORD.
Joshua 7:1 But the children of Israel committed a trespass in the accursed thing: for Achan, the son of Carmi, the son of Zabdi, the son of Zerah, of the tribe of Judah, took of the accursed thing: and the anger of the LORD was kindled against the children of Israel.
Also note that the LORD made a covenant with Israel, and it seems as though Achan was the only one who stole of the accursed thing in chapters 6-7.
So when one person broke the covenant with the LORD it says the children of Israel committed a trespass in the accursed thing.