If the speech can foreseeably be seen as being objectionanable, yet the business is not, the billboard company or the city, or both, can just limit the amount of time it's shown and rotated through in a specified time frame. Just charge them less than the industry standard times.
They can just chalk up the decision to limited ad space. Prove them wrong. Picture a newspaper with ads on one page, some are big, some are small. E-Billboards are just electronic ad space. They can't all be the same size.