Messing with the height of someone's seat by pulling it out before a race would ... not be seen very well. A lot of people can tell when it's off by a few millimeters.
Bike seat height adjustments are very simple: the seat is mounted on a tube, which fits within the slightly larger seat tube and then is clamped.
Getting the exact same position is easy: just use a Sharpie marker on the inner tube to show where it fits in the outer tube. Draw a second, vertical line across both tubes to make sure the orientation is the same.
Yeah, but if you do that before a race... someone's going to do it quickly and not do a great job. Like I said, it's not that difficult to detect a difference of a few millimeters, and it's annoying.
But yeah, they are doing inspections these days.