bash -c 'sleep 5 &' vs. ssh user@host 'sleep 5 &'

[Clark Wang]

In an interactive shell (in case this matters), run bash -c 'sleep 5 &' and
it exits immediately without waiting for the sleep to complete. But ssh
user@host 'sleep 5 &' (with bash as the login shell) would wait for sleep
to complete. Why the difference?

Thanks.

-clark