There are often nation-wide or region-wide advertising campaigns that proudly display a price. If individual cities have different sales taxes, that would make it hard. Personally, I still don't find this to be a good reason. Just charge a single amount anyway and eat the costs in the high tax area. Price it in.
Edit: now that I'm thinking about it, many chain businesses are franchised. So each McDonald's (for example) is independently owned. So they couldn't just eat the cost in those areas.