To briefly answer downbeat's question, it is political, but philosophical (from an engineering standpoint) too. Okay, that was the brief part.
For those of you not familiar with TV station contours (as seen on maps) it is very important to note that they are not
", which usually exceed the actual contour in some or all directions. When a new TV station is being planned, its contour calculations start well before the application for a license. The owner contracts a broadcast engineering firm to do the technical work, and the process usually begins with a simple "ideal" coverage area based on marketing and/or consumer research drawn on a map and handed to the engineer. That is not the contour, it is a "want to have" coverage map.
From there the contour is calculated based on actual distances but also on a variety of other factors, chiefly the surrounding topography, the HAAT (Height Above Average Terrain) of the transmitting antenna, the directionality of the antenna, beam tilt, etc. to arrive at an ERP (Effective Radiated Power) level and its expected field strength in dBu. The broadcast engineer must follow the regulations of Industry Canada and if in a border area the FCC too.
Unlike the older analogue NTSC stations in the U.S. where Class A stations could belt out whopping amounts of ERP to cover huge areas, Canada stayed away from that philosophy with only a few exceptions (CN Tower, major prairie cities). The engineering philosophy was not much different from the concept of firing an old howitzer and hoping for a direct hit.
Now that we're in the DTV age there has been an effort on both sides of the border to force the contour thresholds lower and with greater precision for the sake of energy and reception efficiency. DTV allows that kind of formulation, and the engineering philosophy is now towards "right sizing" contours to their desired audience coverage area.
So, while all the technical stuff is being hashed out, the political side comes in. The CRTC approves/disapproves applications and can send them back to the drawing board during the process. In Canada, there are several good examples of cities for which the contours of neighbouring stations are tightly regulated so as to allow realistic advertising revenues for each: Victoria/Vancouver, Hamilton/Toronto/Kitchener-Waterloo, even Ottawa/Montreal to some extent. Not only is the field strength capped, but the shape of the contours is also highly regulated. The Canadian authorities have often objected to U.S. stations' contours having overlap into Canada, and have intervened successfully to have many such contours reshaped to avoid that.
So you see, setting such limits on the contours of stations is one of the simplest ways to enforce such political considerations and policies from the very start of a station's application process.
My own knock on the capping and shaping of contours is that we've been seeing more and more evidence that DTV-era field strength calculations are proving to be too conservative for satisfactory OTA reception.