It has to do with the metering tab. The tab on the D7000 (as well as the D700, D300, D200, D1, D2 and D3 series) cannot be flipped-up out of the way like it can on the F3, F4, FM/FE, etc. Attempting to mount a non-Ai lens on a D7000, D700, D300, D200, D1, D2 and D3 series may break the tab off. The good news is that John White of Ai Conversions can convert non-Ai lenses for pretty cheap, and he does a good job. Here is his Compatibility Table.