This oversimplfies things a bit. Law firm associates generally receive a salary and a bonus. The bonus can be substantial (six figures) if they hit various performance targets. These targets are largely based on the number of hours billed, although work quality is also relevant.
Law firm partners also don't generally make the same money on their 80th hour. Partnerships will split up the profit at the end of the year, and the number of hours billed and size of the partner's "book of business" factor in heavily here. The size of the book of business depends on the number of hours the partner bills (overseeing associates and paralegals, as well as doing independent work), so working more will increase a partner's share of the annual profits.
Also, a partner whose time is in high demand can raise rates (sometimes done by reducing discounts or charging clients a "NY rate"). So a partner who has plenty of work and doesn't mind not having more work can simply raise rates and make the 80th hour more expensive for clients.
Law firm partners also don't generally make the same money on their 80th hour. Partnerships will split up the profit at the end of the year, and the number of hours billed and size of the partner's "book of business" factor in heavily here. The size of the book of business depends on the number of hours the partner bills (overseeing associates and paralegals, as well as doing independent work), so working more will increase a partner's share of the annual profits.
Also, a partner whose time is in high demand can raise rates (sometimes done by reducing discounts or charging clients a "NY rate"). So a partner who has plenty of work and doesn't mind not having more work can simply raise rates and make the 80th hour more expensive for clients.