Why the university officials do not take action over this copyright violation?
Because they can't. There is no way to tell whether a given electronic preprint violates the publisher's copyright-transfer agreement or not. Different publishers place different restrictions on authors' rights to redistribute their papers. Some allow posting pre-edited versions; some allow posting the official camera-ready version; some allow neither; some only require an exclusive publication license and leave copyright in the author's hands. These restrictions change over time, and may depend on whether the author paid an open access fee to the publisher. The only way to determine whether an electronic (p)reprint is posted illegally is to read the actual copyright-transfer/publication contract. But this contract is directly between the authors and the publishers; universities have no record of these agreements.
Because they don't have to. At least within the US, university web sites generally fall under the "safe harbor" provisions of the Digital Millennium Copyright Act, which requires copyright holder to request removal of the specific items to which they claim copyright.
Because the publishers don't care. Scary legal language notwithstanding, academic publishers in many fields have zero interest in pursuing legal action against individual researchers for posting copies of their own papers in violation of copyright transfer agreements. (I have heard this said specifically about ACM, IEEE, SIAM, and Springer, by people with connections inside each of those organizations.)
Because it's not in their best interest. Both universities and the public benefit materially from the public availability of research by their faculty, students, postdocs, and other researchers. Universities have no incentive—aside from a potential legal threat that they know will never materialize—to proactively censor that research. Many universities, and more recently many governments, have adopted open-access policies that either encourage or require their members to amend publication agreements and make their work publicly available.
Because researchers would revolt. Even if academic publishers started sending DMCA notices to universities, and even if universities required their members to take down copies of their papers, in violation of research community expectations, the people whose research is being censored would simply take their business elsewhere. Those publishers would receive fewer papers, and those universities would receive fewer graduate school and faculty applicants.
