The balancing machine does not balance the wheel along the horizontal axis. It only balances along the vertical axis. (Horizontal = outside edge of rim to inside edge of rim) (Vertical = Top of rim to bottom of rim)
If you watch someone balancing a tire, they usually hammer the weights on the outside of the rim. If the wheel was perfectly balanced on the horizontal axis, putting the weight on the outside creates a slight imbalance. Not enough to matter.
But if the rim is bent horizontally, the tire shop will probably not see it. And this can create a signifigant problem in steering. Remember, people that work there are usually not experts. They get paid slightly over minimum wage, so don't trust them.
Have someone drive your car at a speed which induces the shaking of the steering wheel. If you can see either front rims "wobbling", replace them.
Next, determine at which speeds the steering wheel will shake. Get the car on the highway, start at 40, and slowly work yourself to 90. Note any and all differences. If it gets worse at higher speeds, alignment/bent wheel is where I would look.
But if the shaking is independant of speed, I would look to the steering column. Usually there is a u-joint, and a couple of other connecters. The might be faulty.
After the road tests, post back.
Scott