systemd-oomd: why swap only?

[Oleg Solovyov <mcpain@xxxxxxxxxxxx>]

man oomd.conf:

> SwapUsedLimit=
>            Sets the limit for memory and swap usage on the system before 
systemd-oomd will take action. If the fraction of memory used and the fraction 
of swap used on the system are both more than what is defined here, systemd-
oomd will act on eligible
>            descendant control groups with swap usage greater than 5% of 
total swap, starting from the ones with the highest swap usage.

Why memory usage doesn't count?
Bug or intentional?