What's all this about channels 30 through 40being ssb only?

I have listened for months and years here, in the U.S.,
and except for channel 38,every channel I hear, including
the upper channeels and channels 39 and 40
are all AM-only,

If you broadcast in SSB on channels 30 through 40 here, you
wil get chewed out by someone or many someones..

Unless you're using channel 38

It seems to me that the "gentlemen's aagreement" is
to use only AM on every cb channel except for channel 38.