This is a commonly held misconception. The direct goal of public health policies in a pandemic is not to completely prevent people from being exposed to the virus. The measures it would take to do that would be prohibitively expensive and largely unnecessary. Anyone who has gone grocery shopping even occasionally over the last 8 months has almost certainly had an active Coronavirus (or a few thousand of them) in their body.
The goal of these policies is to reduce the aggregate viral load of the area in total; inside and outside of people. So a mask mandate (or premises rule, or whatever) that is in effect 100% of the time would be ideal from the perspective of reducing the viral presence of the environment, but we know that the last bit of that benefit comes at rapidly diminishing gains. Letting guests drop the mask for 12% of their day in the park (figuring about 1 hour of total meal time per 8 hours in the park) will reduce the benefit achieved but not by much.
But that's only looking at the direct effects of a strict mask mandate. For WDW, the policy is genius. Keeping people in the parks socially distanced and masked helps prevent the disease, but what helps the most is that such a policy alienates the type of people most likely to bring in the virus. People who travel as if nothing were wrong and hate to wear masks are staying out of Disney World right now and that is the best thing for Disney's bottom line.
You might say that the drop in revenue will destroy them. Or that their choice to disenfranchise super-spreader guests is leading to mass layoffs and damage to the economy, but Disney knows that the situation will resolve and things will return to normal. And while it sucks for the employees laid off, from Disney's perspective its financial goal is to reduce the amount they spend treating CMs who catch Covid from the guests (and paying out death benefits).
