Starting with BuddyPress 1.7, extended support for Akismet‘s anti-spam checking was integrated into the Activity Streams component. If you’re using Akismet, BuddyPress will seamlessly and silently send activity stream updates through for scanning, and prevent spammy ones from being made visible to your community based on the very same discussion settings you use for comments.
Typically, plugins and themes would want to opt into supporting BuddyPress; not the other way around. Akismet is a unique integration situation because it comes bundled with WordPress and is currently the best and most readily available solution for mitigating spam, so BuddyPress has chosen to support it.
If you wish to turn off Akismet integration, there is a checkbox in BuddyPress’s settings to do so.