Rabbi Blass answered this question saying "It is forbidden to watch television, educational or otherwise, on Shabbat because of prohibitions relating to Shabbat. On other days, you have to apply self-censorship to what you watch (including commercials) and not spend quality time that could otherwise be dedicated to Torah study or good works."
Rav Eli Mansour writes, "It is, of course, strictly forbidden to watch television on Shabbat (and even during the week the propriety of watching television is certainly questionable), even if one sets the television on a timer before Shabbat, as watching television is not at all in the spirit of the day." Seemingly, Rav Mansour would say the same issue of "spirit of shabbos" whether it was on a timer or just started before shabbos. Rabbi Hassan echoes the same issue here.
It seems the poskim discussing the question online generally point to a muted TV station turned on before shabbos as being a problem of spirit of shabbos.