Because the student would possibly be counting those hours towards the experience requirements for the issue of an NR? After all, it is dual instruction, and it is done at night.
But they cannot.
The experience requirements for the issue of a Night Rating require 5 hours of instruction/supervision
by an FI at an ATO, and the ATO will have to state that the required experience has been met.
Just because a student may misunderstand what the training they are receiving can be used for does not affect the legality, or not, of that training.
Just because 1 Day, 1 Week, 1 Month, 1 Year or several Years later a student
may attempt to misrepresent their prior training does not affect the legality, or not, of that prior training.
An independent IRI is perfectly entitled to train a candidate towards an IR, and could even get them up to Test standard. However only a percentage (0% prior to the CBM-IR) of that training can be used towards the initial issue of an IR.