Plus, it was Satan who instigated David into doing it.
What you have to understand is that the modern concept of Satan as the devil or a fallen angel is nowhere mentioned in the OT. That view of Satan didn't materialise until the NT.
In the OT Satan can only use his power to the extent to which Jahveh allows him to or instigates him to.
The seeming discrepancy between Samuel 24, where god tells David to take the census, and Chronicles 21, where satan tells him to do it can then be understood.