Because nothing is perfectly flat even in our world -- there's always a small incline. It might not be detectable without instruments, but it will be there. Consider also that even if the original builders settled the spires perfectly upright, building tend to sinks and the earth moves. Unless there's some self-correcting mechanism, there's no way it would remain perfectly level.
Note also that all building tend to sway (in wind, minor earthquakes, etc.). Yes, spirestone is incredibly strong and a spire is relatively short and squat, but it's physically impossible for anything to be 100% rigid. It doesn't matter how tough it was built, something 2 miles high is going to be moving, possibly vibrating back and forth as the waves move up and down the spire.
OK, that's all fair from a scientific point of view, in the sense that Straight lines dont actually exist (even light bends thanks to gravitational lensing and all that) though in the scale and context here I dont think it will become all that relevant. Ive been in building that were swaying and built for earthquake survival, but it wasnt hugely noticeable by normal perception. Similarly Ive done the experiment to push on a brick building and wiggle it, it took lasers to detect.
To your initial question, Id say that it shouldn't have any /more/ curvature than the earth itself, which curves at a rate of 8 inches per mile. Given the extreme engineering and the Spirestone/Unobtanium, I dont expect the material properties to be the limiting factor; I rather expect that to be the very reason they are Spriestone and not granite or marble or any factual material. Had it been some real material, then the curvature would have been a much more important design constraint.On the other hand, the presence of support columns in the Habbles confirms what you say about spirestone not being perfectly rigid, so it could easily arch and sag between then just like normal support beams, and they could even pre-load them for added strength like we do.
Also, fwiw, I am currently doubting that the Spires were built in place or by human means. Im presently theorizing that they were dropped for orbit, or teleported in, or some such. But to be fair, you can blame some of that on the recent Starcraft expansion.