The student was removed from the library and charged with obstruction of police; the issue of punishment for his infraction is in the hands of the legal system.
As for your argument regarding we don't know how strong the student was, perhaps he still needed to be subdued although he was in cuffs and on the ground, UCLA PD Asst Chief Young said the student was not actively resisting the officers:
"Young described Tabatabainejad as a "passive resister" who refused to cooperate with officers. He acknowledged that the student didn't actively resist the officers.
"He was 200 pounds and went limp and was very hard to manage. They were trying to get him on his feet," Young said.
http://www.latimes.com/news/local/la-me-taser18nov18,0,4080498.story?page=1&coll=la-home-headlinesAs for "getting him back on his feet" (which wasn't necessary in order to remove him from the premises) via the use of a taser, that line seems to conflict with what the Chief of the UCLA PD said in the same article:
"UCLA Police Chief Karl Ross said the officers decided to use the Taser to incapacitate Tabatabainejad after he went limp while they were escorting him out and urged other library patrons to join his resistance." (Witnesses btw say the student was already on his way out when officers arrived and confronted him, grabbing his arm which escalated the incident into a physical confrontation.)
So the Chief says the cops used the taser to "incapacitate" the student while the Asst Chief says the cops were using it to "get him on his feet" which is hard to do if he's just been tasered and "incapacitated" according to the Chief. They don't seem to be on the same page.
In any case, the use of the taser clearly did not achieve the claimed desired results: the student could still speak and scream (which attracted a crowd) and either wouldn't or couldn't stand to comply with the police. Yet the police continued to taser him as if they would get different results. Rather than an "inducement" for compliance the tool instead appeared a punishment for noncompliance. Considering that they didn't need him to stand in order to drag or carry him out and the student wasn't actively resisting, the use of the taser appears unnecessary, ineffective, counterproductive and excessive.
But again regarding "punishment," the penalty for not showing ID on demand in the library after hours is to not be allowed to remain in the library. The penalty for obstructing the police is being charged with that offense (as the student was) and it's up to the court to decide and mete out the punishment, not the police.
And as for any possible UC disciplinary action, there's also due process for that. In your opinion apparently, one instance of refusal to show ID in a school library not only warrants expulsion but also the denial of admission to any other college or university. So much for proportionality, if you believe a student's failure/refusal to produce ID as in this instance is such an "inexcusable" and egregious violation of authority that it warrants banishment from all colleges/universities. One imagines you likely were not on a college campus in the 60's or 70's if an instance of refusal to show a student ID has you in such righteous high dudgeon.