I may be reading too much into this, but I think the suggestion here is that the "tele" prefix means "at a distance" (tele-phone = distant voice, tele-graph, tele-port, tele-vision, tele-pathy, tele-kenisis, etc.)
Telemarketing refers, both entymologically and colloquially, to marketing via telephone. No one calls billboards or online ads telemarketing.