This topic was sometimes known as "floormat gate"

or something similar in 2014 and 2015 after SCCA "clarified" a vague rule on loose objects to specifically require removing driver side mats for the 2015 season.
It turned out that MANY serious longstanding autocrossers (including myself) do not want to have to remove mats which are properly secured by high quality oem fasteners. The primary reason is that the underlying oem carpet tends to be easily damaged and/or hard or impractical or expensive to replace. A secondary reason is that we simply don't want to have to clean the carpet for no good reason.

There is no safety advantage to removing mats which are properly secured with oem fasteners, at least in late model cars like my 2015 Mustang and previous 2012 Mustang.
As a result of member input the rule was quickly revised for the 2016 season.
FYI even though the 2016 rules posted on the SCCA site are identified as "draft" they are not subject to change for this year other than typo, etc. correction. Any actual rule change for the future must go through the normal SCCA rule change process for implementation in 2017 or later.
Edit to add in response to Richard's question: The oem mats in late model Mustangs are a "hassle" to remove if you have old whimpy finger muscles.
