The problem is that the people summoned a demon by praying to Aslan wrong. Doing wrong things in his name was the problem, of course, not calling him the wrong name. Altough there was obviously deception at work, a lot of people involved in that still thought that they were serving Aslan.
Aslan near the end of the book cuts it apart by just explaining that he is good and Tash is evil, so what you did for which actual reasons matters more than what you thought or how it looked (hello protestantism), but that already implies some duality between them.
There's also the bit in The Horse and His Boy where Aslan turned somebody into a donkey, and sent them to the temple of Tash where they could have their curse lifted. That's just mighty suspicious. You're going to send the bad guy to the Temple of Moloch, messiah? Really? That's a bit odd.
(I mean, I strongly doubt this implication was the authorial intent, of course. But then, neither are a lot of readings of the Bible.)