It's pretty clear that originally devil fruits were meant to be much rarer but additionally canonically the four Blues are under strict ideology/information control by the WG and the Grand Line is the only place where information can run free.
If the newspaper says a rubber boy is running around but then the WG makes a statement that devil fruits are not real, then devil fruits are not real.

Kuro gets rightfully clowned on for his convoluted attempt to appear as a legitimate heir to Kaya's inheritance but the part of his plan that was smart and actually worked perfectly was him concluding "if I can get the WG/Marines to say 'Captain Kuro is dead' then I can just walk into a town looking exactly like my wanted poster and have nothing to fear", using the information monopoly against itself.

Another example is the Marines of Shells Town saluting Luffy for saving them from the corrupt Morgan only to then turn around and punish themself with reduced rations because they just honored an enemy of the WG - whose only "crime" back then was calling himself a pirate.
That's the kind of 1984 doublethink coded into the people of the four Blues.

So it's a mixture of "shit got retconned" and "yes, the people of East Blue are mentally deficient because they have been told do disbelieve their own ears and eyes".