First of all, your premise is wrong: In tennis, the exact Rule 12 of ITF Rules of Tennis is:
If a ball touches a line, it is regarded as touching the court bounded by that line.
Note that the mark of the ball touching the ground is not even a circle; it is not like there is a moment at which we should take a projection of the ball to the ground and decide. A part of the ball might touch the ground e.g. because of compression caused by the impact, and in that case, the ball is in.
(On the other hand, football (soccer) is a sport with such a rule: The ball is out if and only if its orthogonal projection is outside the lines.)
Now, to the volleyball:
The respective rule (FIVB Official Volleyball Rules 2013–2016, Rule 8.4.1) states:
[The ball is “out” when] the part of the ball which contacts the floor is completely outside the boundary lines
with the FIVB Refereeing Guidelines and Instructions stressing “It is essential to realise the importance of the word ‘completely’”.
So: There is no “perspective” involved at all. You need to determine the part of the ball which contacted the floor, and the decision is based on whether this part was outside the line. Or, to use your words, if any player could see the ball not touching the line from his/her perspective, the ball was out. (However, the fact that someone (including referee) saw the ball “above” the line does not necessarily prove anything.)