There are many such topics and complaints of its omission.
At first it was said to be an error/omission on a program/beta update as the Stop button was also AWOL. The Stop button has made a return, but not the Pause button (crazy) and there seems to be what I would consider unreasonable reluctance to bring it back..
Whilst I rarely use on-demand scans other than in beta testing, I would be somewhat miffed if I had something urgent come up and wanted to pause the scan only to have Stop as an option. There is no corresponding Restart option so the scan has to be started again from square one.
I think that the right click context menu scan is somewhat different from the other on-demand scans.