This thread keeps bobbing to the surface, and each time I see it I wonder: are you saying they didn't check in your bike at all, or that they didn't actually take money from you? The border guards never take your money--it's the banjercito which does that part. Some places they're right next to each other, some places they're miles and miles apart in different towns. You can easily blow right by the banjercito, but I assume in such cases they figure it out sooner or later. That implies the question of what they'd do. Forbidding re-entry would be one possibility, but there are others.
Just idle curiosity.
FWIW, I had soldiers at a roadblock check my bike import papers once (out of dozens of roadblocks). This was not at one of those giant barricades with machine guns behind sandbags, guard towers and all the rest--it was on a little highway high in the mountains north of D.F. Probably they were angling for mordida. Mileage varies.