A client is the person a contractor works for. The only client involved is the service itself, as the driver is a contractor.

The person making the purchase is a customer, not a client.

Ideally, you would be right. The customer should not be involved in the business relationship between the driver and the service. However, this is quite obviously not an ideal situation.

With employed waitstaff, tips are, technically, a bonus. A waitress who does not receive enough tips to put herself over minimum wage is entitled to extra pay to reach that point. Legally, this waitress is "adequately" compensated for her time, regardless of tips.

Unlike employed waitstaff, there is no provision to ensure this driver is compensated for their labor. There is no provision to ensure they are reimbursed for their actual expenses. The driver is often asked to subsidize certain customers, by accepting compensation that is less than minimum wage, and even at less than actual cost. They are often asked to pay to work.

Employees are protected by labor laws. When a group of employees decides to demand better pay, their collective action is explicitly protected.

When contractors do the same, their collective action violates anti-trust laws. They are guilty of collusion and price fixing.

The services have found a legal loophole to evade minimum wage and labor protections, and they are using it to exploit drivers.

The customer benefits from this. The customer is complicit in the exploitation. Ignorance does not negate their complicity.

(You should also know that if you do tip well, the services reduce their base pay further, effectively capturing part of your tip to the driver. They are exploiting you, too.)

Again, nobody should be using these services. They are bad for everyone. The only ethical way to use them is to tip sufficiently to ensure the driver's pay exceeds minimum wage.