ggplot2 geom_abline slope not working?

I am learning ggplot2, and need your help.

When I try

p <- ggplot(mtcars, aes(x = wt, y=mpg)) + geom_point() 
p + geom_abline(slope=5) 

(from http://had.co.nz/ggplot2/geom_abline.html)

the slope of the abline does not change, but this works:

p + geom_abline(intercept=20)

In order to have slope work, I have to use

p + geom_abline(aes(slope=5)) 

Is it a bug, or is there some reason slope and intercept were treated
differently?

Thank you very much in advance for your clarification.

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