I think that this use is fine, and some professors may add their subspecialties to these skill lists (e.g. Algebraic Geometry). I think that being prescriptivist about how a social networking site is used is a bit fallacious in the first place. Professors may use LinkedIn to find connections that lead to work in similar ways to non-academics. There is no "supposed to be" in this context.
For example, at most US universities faculty are responsible for finding funding for their summer salary. That might come in the form of teaching summer courses, finding grant funding, or working as a consultant to a business or national lab. Putting oneself out there on LinkedIn might be a great way to find or be found for consulting gigs. Also, not all profs are permanently committed to their current jobs, so being on job-based social networking sites may lead to their next opportunity.
I agree that it's a little silly for senior people to be listing skills like this and farming endorsements, but I think this is more like the future than it is like the past. I wouldn't be surprised to see LinkedIn add features that make it more attractive to academics to use. The site is already an online resume for non-academics, adding academic-style publication references, grants, courses taught, etc. might allow us to use LinkedIn as an online CV. The site already has publications, but they don't really support them like ResearchGate, Google Scholar, or other academic publication aggregators (yet).