Bug or expected behaviour

Could someone pls explain me the following,

I have two table tables emp and dept

emp
----------
empid
empname
dept_no
age

dept
----------
deptid
deptname

billdb=# select * from dept\g
deptid | deptname
--------+----------
100 | Oracle
200 | PHP

(2 rows)


billdb=# select * from emp;
empid | empname | dept_no | age
-------+---------+---------+-----
1 | Abhay | 100 | 24
2 | AB | 200 | 26
(2 rows)



billdb=# select * from emp where empid in ( select empid from dept);
empid | empname | dept_no | age
-------+---------+---------+-----
1 | Abhay | 100 | 24
2 | AB | 200 | 26
(2 rows)


Why the query is returning rows when empid column doesn't exists in dept
table. Should throw an error.

Thanks
Ravinder