Indeed. Because these were all scheduled operations until well into the mid/late 1960s. Airlines like KLM and SAS got traffic rights through Prestwick and elsewhere (eg Manchester) during prop aircraft stopover days and didn't want to let go, until it was reasoned that the small numbers of passengers boarding didn't cover the costs. They were only a small proportion of their total transatlantic flights. They were often flights to secondary US points like Chicago.
Pan Am (DC8-30s by the way, the first ordered) had a daily service through Prestwick.