The cheapest units are wired units, with a magnet based sensor that goes on the wheel, and the designs are such that the head-unit (the display) is fixed to the handlebar with zip ties, that is, it's not designed to be removed often.
Now, though the head-unit is nearly useless without the sensor and magnets, most potential thieves don't know that, and they end up stealing them anyway, as zip ties are not really theft prevention devices. A wireless head unit like a simple Garmin Edge 200, with a quarter-turn mount, is easy to install, easy to slip on and off the bike, and super easy to use. The mounts themselves aren't attractive to theives, and if you have more than one bike, you can install extra mounts on the other bikes and use the same unit across the bikes, not something you can do with the cheaper wired units unless you buy extra sensor and cable sets, which is nearly as bad as buying the whole set again. You can even use the recorded activity files for uploading to strava etc.
/Prashanth