Why would a tenure-track job posting ask for an undergraduate transcript?
I had to submit undergraduate and graduate transcripts to several schools each time I applied for jobs in mathematics. I believe the main reason is simply to allow the school to verify that you really have the credentials you claim. 

There have been examples where faculty and administrators were caught claiming credentials that they did not possess, so schools are more cautious about such things now. The same is true for background checks - many schools now run a background check before they make an official job offer.  

At my institution, when we have a mathematics job search we don't really look at the grades on the transcripts. The cover letter, reference letters, teaching statement, and research statement are scrutinized, but the transcripts just get a cursory glance to make sure the person has the right coursework (e.g. if we want to hire someone in a specific subfield, we might check that they have coursework in that subfield).