In the 2nd verse in Shelach, it says שלח לך (Shelach Lecha), which Rashi explains as " לדעתך, אני איני מצוה לך". Why don't we give the same explanation the other places in the Torah it says לך. E.g. in Parshas Lech Lecha, or by the Ketores (קח לך)?
It has to do with the fact that in this case, the same incident recorded later in the Torah, seems to tell us that it was Moshe's own decision (and the populace's initiative) to send the mission. We do not have such exceptional circumstances in the other cases you mentioned.
Edit: Actually, this only answers why Rash"i does comment here. The truth about why he doesn't comment elsewhere is probably just because a verb with a purposeful prepositional pronoun is a standard way to express a certain type of imperative in Tana"ch. See this interesting hirhurim post on the "polite imperative" as a description of this form.
The Maskil Ledavid says that Rashi actually learns that it was Moshe's choice from the word "Shlach", not "Lecha".
He says Rashi had a question. Shlach would normally be a command, and if so, what was the purpose of the command? Also, how could G-d command Moshe to do something that would end badly (since G-d new what the spies would do)?
To answer this, Rashi says that Shlach here means if you want to, it is not a commandment, as it would normally be.
Also, the Maskil LeDavid says that Rashi learned out from "Lecha" why this story is said right after the story of Miriam. G-d was basically saying that the spies should learn from what happened to you (i.e. Miriam got punished for saying Lashon Harah about you), and they should realize that saying Lashon Harah about the land would be a bad idea.
If Rashi already uses the word "Lecha" to learn something else, he must learn that it was optional from the word "Shlach".
See there for more details.
I second Ariel K's answer, that it is an ambiguous phrase, which is darshened according to context and purpose. Midrash is really not that systematic.
But to take the answer in the opposite direction, Shadal actually takes the derasha of לטובתך, from Lech Lecha, and applies it here. See my presentation and translation of Shadal here.
But this is something that many meforshei Rashi would address. For example, Gur Aryeh says that letovatcha == ledaatcha. If you are up for it, you can run through the various meforshei Rashi in my source roundup on Shelach and see what they all have to say on this topic.
Just another great answer I saw that I must Share.The Ohr Hachaim Hakodosh on the question of why such Great people as the Meraglim that the torah says when they left where Tzaddikim became so evil so fast (excluding the Teretz that they wanted Moshe to live Longer And they where afraid of not being able to live on a close level to Hashem when not living off Mon meaning afraid of becoming "Kochi Votzum Yodi")So he answers that "Shliach Shel Adom Kimoso" and granted they where Tzaddikim but once you are doing the Shlichus for Klal Yisroel You are going to get there Bad Habits too and The Chasam Sofer builds on this Yesoid and says thats why it say Shlach Li'cha because if not they will get the traits of Klal Yisroel instead of getting the Shlichus of Moshe and getting his Tzidkus and we see what the outcome was.
There is rather a long and interesting Ohr aHaim on the word "lecha". I will make it very simple and short. 1."lecha" means permission and not command. 2."lecha" refers to "anashim", i.e. when they are close to you, Moshe, - they are tzadikim but when they are away from you - they are reshaim. 3."lecha" gives a hint that God wanted that the whole meraglim mission should seem to be initiated by Moshe and not by God! 4."lecha" means that the goal of the mission should be like you think of it and not like people think. 5."lecha" means that Moshe will get extra 40 years of his life, so this is 'lahanatecha" like in Lech lecha. I guess this is it. Id someone is interested in details my advice is to read Ohr aHaim aKadosh!