Interestingly, I've always interpreted the spell such that the target can twist upside down. I read the term "suspended" in the spell description as applying to the entire target. And a logical consequence of suspending the entire target is that they can rotate.
Sure, you could read the spell as only suspending part of the target, or as suspending the entire target but also preventing rotation, but either option would seem to be reading more effects into the spell than the description includes.
It seems like we're taking different approaches to the idea that spells do what they say, and nothing more. If I understand your position correctly, you're saying that since the spell is silent about allowing the target to rotate, the target is unable to rotate any more (or less) than someone standing on the ground. From my standpoint, the target is unable to rotate any more (or less) than someone entirely suspended in mid-air by some means other than the Levitate spell. We're just picking different baselines.
By way of analogy, the Banishment spell doesn't say anything about permitting the target to rotate. But if the target is Banished to Limbo, they certainly can rotate while the spell is in effect. I would argue that the appropriate baseline for whether the target can rotate is "someone else in Limbo" rather than "someone on the Prime Material who wasn't Banished to Limbo". In other words, the target's new ability to rotate freely is a consequence of being on Limbo, rather than a spell effect. Similarly, I view the ability of the target of Levitate to rotate freely as a consequence of being entirely suspended in mid-air, rather than a spell effect.