They do it because they want the video to be synchronized with the slide show presentation on the main part of the page. By using a single control at the bottom of the page, they can more easily keep control of your interactions, as opposed to separate slide show and video controls. In fact, since the video is in an iFrame, no changes to the video scrubber can be detected on the main site. As a result, they will not be able to match the current presentation slide to where you are looking in the video.
Another possible reason is that the video is just too small to allow for effective searching, that the touch zone would be too difficult to reach and that the slightest drag would result in significant jumps in playback.