YTD always downloads the highest-quality video it can find. Basically, that can mean one of two possibilities for Alois Nebel: 1) SD stream is the best quality available. 2) There's a bug in YTD which prevents a proper identification of the best stream. In either case, there's not much you can do. You could try to download the video from the command line, because then YTD will print out a command which would let you download the video "by hand", using WGET, MSDL or RTMPDUMP (as appropriate for that particular video); you could then try to play with the parameters (e.g. if you saw something like "Video_240p.mp4", you could try to change the command line so that this parameter read "Video_720p.mp4" and maybe you'd get lucky - some providers are more tolerant to this kind of manipulations that others, and if I remember correctly, CeskaTelevize.cz is one of those more tolerant).
If you post a link, I can check for the best quality by hand.