In the introduction to Tractate Shabbat in his Commentary to the Mishnah, Rambam writes:
אבל מקום פטור מותר להוציא ממנו לכל אחת משלש הרשיות ומהן אליו וזה מותר לכתחלה ולפיכך נקרא מקום פטור
But a mekom p'tur it is permissible to take out from it to any one of the [other] three domains, and from them to it, and this is permissible ab initio and therefore it is called "a place of exemption".
Here Rambam tells us that the reason for the name mekom p'tur – place of exemption – is that carrying to and from that domain is entirely permissible. However, this seems to be a puzzling reason to give the domain this name. In the area of Shabbat, the term "exempt" (patur) usually carries the implication that the perpetrator is merely exempt, but not that the act is entirely permissible:
והאמר שמואל כל פטורי דשבת פטור אבל אסור בר מהני תלת דפטור ומותר צידת צבי וצידת נחש ומפיס מורסא
did not Samuel say: Everything [taught as] involving no liability on the Sabbath, involves [indeed] no liability, yet it is forbidden, save these three, which involve no liability and are [also] permitted: [viz.,] the capture of a deer, the capture of a snake, and the manipulation of an abscess?
Given that the connotation of "exempt" is expressly not "entirely permissible", why would this domain be named "a place of exemption" specifically to illustrate that it is entirely permissible?