It's been like this since the start. You have to keep your signal in the authorized band. If that means you have to 'scootch' up or down a little, then just do it. It also assumes that you know signal bandwidths, band allocations, etc. Why isn't it the same on the 'CB' channels? It is, but since the center frequency of each channel is determined for AM (3K of signal on each side of the center freq.), and since a SSB signal should only be 'half' of an AM signal (upper or lower) then you are still within the allowable channel dimensions/allocation. (Uh... ~if~ the approved equipment hasn't been mod'ed, right?) Also why the 'CB' channels have the spacing that they have. A good SSB 'rule of thumb' is to stay 3 Khz inside the band edges. That '3 Khz' thingy depends on which side band is in use, but sure makes it easy to remember (and so you loose about 2Khz on one end or the other, big deal).
Different modes have different band widths. CW is about .5 Khz, AM about 6 Khz, SSB about 3 Khz, etc, etc. (Hint: the mode designater, 'J3_ ' for SSB for instance... the number is the signal bandwidth.)
- 'Doc
All this stuff used to be on the test at one time or another. 'Extra' stuff to remember, but it kept those 'pink slips' out of the mail box... well, most of the time.