I asked this same question on an email list after the CQ WW SSB contest. The answer is quite simple ... the IARU bandplans are recommendations and are not enforced by all administrations.
For many countries (apparently many in the Caribbean) there are no restrictions regarding what modes modes can be used where so long as the operator is within the ITU band allocation.
Perfectly legal for many of the international stations but perhaps not too gentlemanly IMHO. I heard a number of Caribbean
stations calling below 7.040 in the CQ WW SSB, and listening in the US phone band, but I also heard a number of US Extra Class Amateurs working these guys below 7.040 (ie I could hear both sides of the QSO so they were not working split). US Extra class hams should know better.
I would like to see the organizers of some of these contests put some hard frequency boundaries on operation, with penalty of disqualification. That would be the only way to stop this practice during these big contests ... but I'm not holding my breath.
I realize that this is a touchy topic for many US hams but Canada does have an SSB allocation from 7.050MHZ to 7.100MHZ
and this aligns with the IARU bandplan. The US seems to be one of the few nations that doesn't allow SSB in this subband.
for the IARU Recommendations.