But why didn’t he (Roberts) just reject it like Kennedy did?
As I understand it if Roberts had denied it, Taitz could refile with a third Justice and then a fourth and so on. Scheduling it for the whole Court allows all the Justices to hear it at once and deny it. After that she can not refile it.
Rule 22 of the Supreme Court governs applications to individual Justices.
Rule 22
4. A Justice denying an application will note the denial thereon. Thereafter, unless action thereon is restricted by law to the Circuit Justice or is untimely under Rule 30.2, the party making an application, except in the case of an application for an extension of time, may renew it to any other Justice, subject to the provisions of this Rule. ...
5. A Justice to whom an application for a stay or for bail is submitted may refer it to the Court for determination.
http://www.supremecourt.gov/ctrules/2010RulesoftheCourt.pdf