The reason for the change is almost certainly because these machines now run at cool enough temperatures for a human to use on their lap without injury. Intel

The reason for the change is almost certainly because these machines now run at cool enough temperatures for a human to use on their lap without injury. Intel Macs could reach temperatures around boiling point, so if Apple had marketed them as "laptops" it would have been awkward: "I'm sorry, sir, technically we designed your laptop for use on a flat surface. If using our laptop on your lap is painfully hot, move it to a table"