I emailed Rabbi Eliezer Zalmanov (from Chabad.org) about your question. Here's what I said:
B''H
In most Ashkenazi communities, the custom is that bochurim do not wear a tallis. However, when a bochur is called to the Torah or to the amud, he puts one on anyway, although he makes sure not to cover his head with the tallis so that it's not considered an atifah.
But in Lubavitch, bochurim don't put on a tallis even in the above cases. Bochurim are called to the Torah and the amud and lead without a tallis.
What is the reason for this minhag?
Sincerely,
Ezra
His response sheds some light on this subject:
Hi Ezra,
That's a good question.
The truth is that Lubavitcher bochurim probably should wear a tallis when being called up to the Torah or the amud, because it is a matter of 'kavod hatzibur"--everyone is wearing a tallis so it is disrespectful to the congregation when a person on the bima is not.
The reason it does not happen in Lubavitch is because the main weekday minyan in 770 for many years was considered the "bochurim's minyan." Often the Rebbe would join the minyan for Torah reading and he would receive an aliyah. Because he had already davened, he would not be wearing a tallis, and he would not put one on because there was no issue of kavod hatzibur, because in this case the majority of the minyan was not wearing one.
On Shabbos, though, when the main minyan was the community minyan, bochurim receiving aliyahs then (usually a chosson the Shabbos before his wedding or on rare occasion a bar mitzvah boy) would most certainly wear a tallis.
I hope this helps.
All the best,
Rabbi Eliezer Zalmanov
for Chabad.org