True. But the Justices don’t just say “We ruled this way because we felt like it.” They offer a rationale to justify their decision.
And they tend to be some of the most lucid, logical, and persuasive rationales out there. Anybody interested in this stuff, especially for more controversial topics (free speech, corporate personhood, the gays, etc.), should really read both the majority and the dissenting opinions of the Supremes.
They do not write in legalese and jargon. They write in plain, readable English so that the general public now and later, in history, can understand why certain decisions were made the way they were – really good stuff, and often better explanations of the actual issues at hand than the mangled news stories they inevitably spawn.
In general (and IMO, of course) they are one of the few parts of government that actually thinks things through and does stuff for the sake of history, not an immediate boost in polls. Their writings are not the deliberately dense bullshit of Congress or the flowery platitudes of the Presidency, intended to appease rather than elucidate. It’s really good stuff.
They were a lot more cogent back in the days when the judges had to write them by hand.