Crashing into a ship in orbit takes a lot of doing - like you say, space is big, and if ten thousand starships parked on low Earth orbit today, warping into their midst blind should still be statistically quite safe. You really need to crash into a hostile starship's death rays, or to the dense debris field created by same. And this only happens if the hostile starship knows beforehand where you are going to warp (which Nero has every right to know, as he seems familiar with Starfleet procedure and isn't exactly facing their best and brightest).
It's pretty consistent that things on or near planets are obscured from long range sensors, whereas things seen against the background of deep space are somewhat detectable - and things actively glowing with warp energies can be both spotted and identified far more easily than things not doing so.
I don't disagree that stuff in close orbit should not be easily detectable. I disagree that any ship should be warping in and out so close to any planet, for that very reason. It's similar to driving your car really fast just because you can right up until the point some other tw*t is driving his car really really fast in the opposite direction. They don't even seem to have air traffic control in Star Trek and they certainly had no input from Vulcan. Anything could have been waiting and - oh - was waiting - lol.