They do have promotional fares but they are extremely limited.
If you take your reasoning further, then if they sold every fare for $1, yield would be increased.
A scenario of a basic 500 mile flight.
100 seats for sale, but only sold are the following;
5 at $30
10 at $50
50 at $100
65 seats sold, at a total revenue of $5650, and an average fare of $86.92. Revenue Passenger Miles is 65 x 500 = 32,500.
The yield is 5650/32500 = $0.174 cpm
Repeating the above, but forgetting about the "loss leader" $30 fares.
10 at $50
50 at $100
60 seats sold, at a total revenue of $5500, at an average fare of $91.67. Revenue Passenger Miles is 60 x 500 = 30,000.
This time, yield is 5500/30000 = $0.183 cpm
Lower revenue, lower load factor, higher average fare, higher yield.
This excludes costs which obviously need to be accounted for...yield on its own is not the best metric as it's hard to make comparisons without a lot of other data. But this was just to make the point.