Because back in 1994, Mazda put in the US owners handbooks as long as you didn’t use more than 10% ethanol (gasahol), your engine warranty would not be voided. In the litiguous US, that had meaning.
Back in 1993, or perhaps earlier, Mazda had already done the tests. No need to repeat them. They must surely know what is the difference, if any, between the various markets. In fact, back in 1996, if you needed a part for a Eunos Roadster from Mazda, the approach was to go to the dealer. The Dealer sends a fax request to Mazda Cars Limited, who’d shrug their shoulders, and forward a fax to Hiroshima. Someone who understood English would translate it, and then, someone would cross reference the part number.
Notably, Honda say E10 is fine with any Honda with fuel injection. Toyota; mid-90s i believe. These companies haven’t done new testing to satisfy a market of 450 million souls. But dug out the same lab results they performed decades ago. Gasohol was introduced in 1979…
As it is, Mazda Japan issued wording, copied verbatum by the UK government, that implies any Mazda made since 2002 is ok with E10. Mazda UK tell me that’s not the case. So much for issuing litigiously conservative (play it safe) statements.