On Nikon DSLRs with built-in flash units, the flash mode setting is automatically switched based upon how the camera's metering is set. If you set spot metering, the camera switches to Standard TTL (note: a few much older DSLRs and some film SLRs also set Standard TTL when you use Manual exposure mode).
This, by the way, is probably the wrong design decision, especially now that we have so many autofocus sensors to which spot metering can move in some focus modes. You have to be very careful about where you place the spot—because the ambient metering will also be done on that spot—which is one reason why I've backed off a bit on my suggestion of using Standard TTL instead of TTL BL on bodies with internal flash.
