Assuming that the spell in question targeted as part of it's casting, this seems to be a rather unusual, and probably incorrect implementation of Rule 601.2 and 601.2c, and completely ignores 721.1.
Normally targeting an illegal creature would just roll back to before the casting started, but the combination of the 'unable' in 601.2's "If a player is unable to comply with any of the steps listed below, the casting of the spell is illegal;" and 601.2c's "The player announces their choice of an appropriate object or player for each target the spell requires" could be taken to mean that once a casting was started a play would have to complete it if there were valid targets.
However, this reading of the rules would need to completely ignore 721.1's "starts to take an action but can’t legally complete it, the entire action is reversed and any payments already made are canceled"
However, if the spell you cast did not target as part of resolution, and rather targets as part of a triggered ability (e.g. Oblivion Ring), the judges ruling would more than likely be valid.
Rules in question:
601.2. To cast a spell is to take it from where it is (usually the hand), put it on the stack, and pay its costs, so that it will
eventually resolve and have its effect. Casting a spell includes
proposal of the spell (rules 601.2a–d) and determination and payment
of costs (rules 601.2f–h). To cast a spell, a player follows the steps
listed below, in order. A player must be legally allowed to cast the
spell to begin this process (see rule 601.3). If, at any point during
the casting of a spell, a player is unable to comply with any of the
steps listed below, the casting of the spell is illegal; the game
returns to the moment before the casting of that spell was proposed
(see rule 721, “Handling Illegal Actions”).
721.1. If a player takes an illegal action or starts to take an action but can’t legally complete it, the entire action is reversed and any
payments already made are canceled. No abilities trigger and no
effects apply as a result of an undone action. If the action was
casting a spell, the spell returns to the zone it came from. Each
player may also reverse any legal mana abilities that player activated
while making the illegal play, unless mana from those abilities or
from any triggered mana abilities they caused to trigger was spent on
another mana ability that wasn’t reversed. Players may not reverse
actions that moved cards to a library, moved cards from a library to
any zone other than the stack, caused a library to be shuffled, or
caused cards from a library to be revealed.