If the client didn't agree to it, then it's not ethical to charge them for it. What this comes down to is the client is effectively presenting you with only 13.5 hours of demand. This decrease is due to them making less work possible rather than explicitly telling you that there's less work that they want you to do, but it ends up with the same effect. So ultimately, your complaint comes down to a client not assigning you as much work as you want, and you wanting to bill them for how much you wanted to work for them, rather than how much work they presented you with. You say this affects your ability to forecast revenue, and that raises the question of just who should bear this burden.
Variations in demand is a common phenomenon. Just who absorbs that variation depends on the relationship. In an employer-employee relationship, the employer absorbs the variation; if there are times of days where a supermarket has half the customers, they don't pay their employees half the hourly rate. With a customer-vendor relationship, the vendor absorbs the variation; if a hotel has a period of low vacancy, it doesn't send its previous customers a bill for the unused rooms.
Relationships between clients and contractors exist on a spectrum between these two extremes, and you need to figure out where you want to be on that spectrum, communicate that to your clients, and get them to agree. There are trade-offs to this decision. Part of the reason that the nominal rates of contractors tend to be greater than those of employees is that the costs of this variation is priced into their rates. In your own words, this client is paying a "fairly lucrative rate". Asking your clients to pay lucrative rates and expecting them to pick up the tab when demand goes down is double dipping. If you want a more regular income stream, you need to work out how to put conditions in your contracts to achieve that, and be prepared for the amount you can charge to decrease.
