Gavra:
I know my statement is provocative, but here goes. You stated that the Roshei Yeshivos, by not expelling bochurim who smoke, must hold that it is muttar. You draw distinction to how they would handle someone who has unfiltered internet. You are correct. If questioned, I dare say that there is not a single Rosh Yeshiva that could justify being more stringent about internet than tobacco. If anything, smoking is a definite damage, and the issur is based on a certain result. Internet, as dangerous and risky as it is, has applications that are kosher. Poskim are correct in requiring filters and monitors. But the risk is still a percentage (albeit a big one).
What is at hand is that these same Roshei Yeshivos spent their learning years in yeshivos that did not assur smoking. My own experience was returning to the beis hamedrash after the morning shiur for mincha, and finding a thick cloud hovering from the ceiling down to a few feet from the floor. The issur is viewed as a chiddush, and not as accepted. Internet was never accepted as muttar, so they are less tolerant.
To me, the distinction is one of ignorance. Sorry that I speak thus of Roshei Yeshivos.