Determining set of states that satisfy a CTL formulae
The semantics of the "Until" operator allows for the eventuality to be satisfied immediately. That is, $pUq$ holds both in the computation $ppppq$, but also in $q$.

Thus, in your formula, the eventuality is satisfied immediately in states $s_1,s_3$, and satisfied within 1 step in $s_2$. But all of them satisfy it.