Yeu, sync has ads because the developers of sync put ads into it.
Vanilla lemmy and jerboa certainly dont.
I can imagine some instance admins considering forking lemmy and putting in ads. At the moment, most instances are run as a hobby (for free) or by contributions (ie donations from members).
The development of lemmy was funded by a VC-type thing for a while, not sure if its still active. Otherwise, its also developed thanksnto contributions