I really don't see why it matters. I mean, the cover looks nice, that's all that matters and it vaguely touches the subject. It's not exactly like Stargazer has anything to do with an observatory, either.
Difference is I don't think anyone cares about whatever Stargazer IS about, whereas Dune fans can be fucking nuts.
Stargazer is actually one of my favourite Dio written songs, but truth be told, the story in the lyrics was a bit too esoteric to be summed up in a single image, so the band chose a pic that was a literal representation of the song title. It may not have been the most creative cover for the single, but it still related to the song at least in name, if not in spirit.
To Tame A Land is a wholly different creature. Dune is one of the best selling works of science fiction literature in the history of the genre, considered by many to be the sci-fi counterpart to The Lord Of The Rings, and has spawned a multi-media franchise of numerous sequels, prequels, films and video games, with an influence that can be felt in many of the most prominent sci-fi works that have followed it, from Star Wars to Warhammer 40000 to Avatar. A song adaptation of such a work is inherently subject a much higher level of scrutiny concerning it's loyalty to it's thematic basis, and physical representation of such a song is subject to the same. Granted, what the band chooses as the image for a cover single is not all
that important, but when that song is based off a franchise like Dune, people are going to expect some sort of meaningful visual tie-in, rather then just some random pic of unrelated creatures in a wide open space vaguely reminiscent of the primary setting of the book.
(It should be noted that the cover of TTAL roughly relates to the literal title of it's respective song, although nowhere near as well as the cover of Stargazer)
Edit: And while I think the mega-uber-remote chance of this left with Portnoy, I'd bust a nut if they ever covered this live and followed it with March of the Tyrant. (which is about a later Dune book)
Where did you hear that MOTT is based off a Dune book, because if that's true, then it's badass!