That's been an Olympic sport since 1904, descended from a popular English pastime.
In that sport, you're supposed to always have one foot touching the ground. Then the cameras with the slo-mo showed that 100% of the "walkers" weren't following that rule, that all of them had both feet off the ground at some point. So rather than enforce the rule strictly, they wink at a little bit of cheating. It's a judgement call by the officials about how much "both feet are off the ground" cheating is allowed.