Because only the elect know the voice of the Holy Spirit and will not be operating through human perception of events.
This sounds like an overly spiritualized way of saying that, so long as you have good intentions in your heart, you can't be deceived, which is exactly what it takes to be deceived.
I agree that it is impossible for the elect to be deceived, but one is only "elect" insomuch as he follows God's spirit. Jesus said that his teachings are his spirit. He said that the job of the Holy Spirit will be to remind us of his teachings.
The mark prophecy very clearly states that the purpose of the mark will be to control buying and selling. Any interpretation which does not include buying and selling (i.e. money) as a central, integral part of that interpretation will be inaccurate.
Now, compare this with what Jesus said about our relationship to money. He said we cannot work for God and money at the same time without cheating on one or the other. He said we should not let worry about food and clothing stop us from stepping out in faith. He said all the nations of the world chase after these things, but that we should not be like them. He said that we should seek his Kingdom and God, who knows what we need, will take care of us.
There is a central theme both in the mark prophecy and in the teachings of Jesus regarding our reliance on materialism vs our dependence on God. In either case we must make a choice. As it is now, a person can still work for money at pretend to be a follower of Jesus, but a time is coming when the choice will be made much more clear; a line in the concrete. If you take the mark in your hand so that you may continue to buy and sell, you are making a declaration as to whom you really serve.
People who disregard or argue-away this important choice are almost certainly doing so because they themselves have not confronted their reliance on materialism. They want the salvation Jesus promised, but they do not want his teachings. They do not want to reject the system which feeds them so they will invent all kinds of convenient doctrines to make themselves feel better about their reliance on money.