That is the separation of Toshu and Tanto randori...as Wolput Sensei explained.
In addition, in Tanto Randori, the roles of uke and nage are defined, and instantly reverse in certain situations.
Toshu Randori provides a much more assertive shiai.
I see, but what I am really trying to wrap my head around is why the tanto is ignored after the first strike. When my school does tonto randori, the tonto can uke can attack again if the first one fails. The uke is at a disadvantage if the have to keep a hold of a knife but not use it because they can't use their other hand (I am assuming that uke can throw nage in this situation to get a point...)
Why keep the form of knife fighting for the first strike and then switch to unarmed tactics without a definite disarm as apposed to keep treating the attacker like they are armed or constantly treating the uke as they are unarmed, I guess would be my question.