IndyCar have released a statement on their Facebook page, regarding Pagenaud's penalty.
https://www.facebook.com/indycar/?fref=nf
INDYCAR Statement on Pagenaud Penalty:
Simon Pagenaud’s actions during the Toyota Grand Prix of Long Beach were deemed an infraction per Rule 7.10.1.1. “Lane Usage” of the INDYCAR Penalty Guidelines: Failing to follow designated procedures entering or exiting the pit area, including the proper use of the acceleration and deceleration lanes. The penalty for this infraction ranges from a warning (minimum), putting the driver to the back of the field (mid) and drive-through or stop and go/hold (maximum). INDYCAR race stewards determined his actions were not deemed severe enough to warrant a harsher penalty than the warning that was issued.