This question is good, first, if you look carefully the Rashi, you will realize that the person in question does not accept to distinguish between Samaritan and Jew concerning the Mitsva to return. So, if he returned the Samaritan's lost by extra duty (in a duty free way) there is no problem.
The words of Rashi are :
והמחזיר אבידה לכותי - השווה וחבר כותי לישראל ומראה בעצמו שהשבת אבדה אינה חשובה לו מצות בוראו שאף לכותי הוא עושה כן שלא נצטווה עליהם:
He decided to make an equivalency and a bound between Samaritan and Israeli. He clearly shows that he refuses to consider "Hashavat Aveda" as his creator's order. He demonstrates this when he returns the lost of the Samaritan while he has no "Toraic" obligation to return them.
Not required I hear, but forbidden?!
Rambam (see Kessef Mishne for sources) said clearly that if by performing Hashavat Aveda to the Samaritan you will make a kiddush Hashem and avoid a chilul Hashem, then these would be reasons to return it. The reason for the early prohibition is that Tora doesn't regard the money of a gentile as dedicated to a good thing regarding avodat Hashem. They are not fascinated by avodat Hashem.
Not required I hear, but forbidden?!
Here you mean he is exempt from duty, and is forced to stop the instinctual act of returning. Because the mitzvah of hashavat aveda has a rationale beyond simply returning everything to the owner. The general rationale is to help him to continue to serve his master (Hashem).
We learn this from our Gemara:
והמחזיר אבידה לכותי עליו הכתוב אומר (דברים כט, יח) למען ספות הרוה את הצמאה לא יאבה ה' סלוח לו
One who returns a lost to a Samaritan, Tora says about him "to add drunkenness to thirst"
Rashi:
רוה - עובדי כוכבים ששבעים ואינן צמאין ליוצרם צמאה זו כנסת ישראל שצמאה ותאיבה ליראת יוצרה ולקיים מצותיו:
Idolaters have more than enough of their creator. Jewish people is called thirsty, because it is thirsty and willing. Id est to maintain a relationship with Hashem, based on fear, and to obey his orders.
The act is the same than the act of mitsva but the spiritual background is not the same.
But how is this different from Loshon Horo (gossip) which is forbidden to speak about a goy
Actually AFAIK there is not particular prohibition of LH against non-Jews.