Airbags deploy with a rather powerful explosion. My assumption is that since these covers don't wrap around the back of the seat, the force of the airbag exploding is enough to push the edge of the cover as the airbag deploys. Meaning, the cover will likely move rather than constraining the airbag. Note that the straps are also not super sturdy and placed well below the deployment area.
That said, this is just based on my observations. You can try reaching out to the manufacturer to see if they have a different explanation...
Hope this helps!
