For many small business owners, they simply don't have time to be looking after a website in the same way you or I would. In many cases, web presence isn't make or break for the business, and a website is just one of many things they do that involve telling a company "we need X", handing over money, and getting X.

If you are the company whose job it is to do whatever they asked for - build a website, see if anything can be done to help improve traffic, install a gas oven - it is your job to not defraud them. Charging them to boost their ranking temporarily, knowing full well it will fall again soon, is fraud.