My grandfather asks thwy was the Torah insistent in this case that the offering be given with a though lishma? He explains the mishkan servd to make up for the sin of the Golden Calf. When it come to the sin of idolatry, Hashem does count the though along with the deed. As they then had a sin of thought on their account, the mitzvah that serves as a kapara has to also encompass thought.
What of the principle presented in Pesachim 8B that someone who offers a coin for charity in order to save heis son or that he should have a portion in the Next World is considered completely righteous? That only pertains to other forms of charity. However, with respect to the mishkan, the principle is "li hakesef veli hazahav eyzo bayis asher tivnu li. [The silver and the gold are for Me for the house that you build me]
Related post http://kallahmagazine.blogspot.com/2015/10/dvar-yehudah-parsha-points-from-my.html