Regulators can always declare a merger to be monopolistic -- they just have to define the market narrow enough. For example, if the FCC and FTC are considering calling satellite radio a separate market from terrestrial radio as an excuse to stop the Sirius-XM merger. The NAB, the trade group fro terrestrial radio, has been going ape trying to block the merger, knowing that the two together will cause its stations to bleed listeners to satellite even faster than in the past. Hilariously, though, the NAB is having to twist itself into pretzels as it goes to Defcon 1 trying to stop the merger by ... arguing that satellite radio is a separate market from terrestrial radio and thus the merger is monopolistic. Begging the question, then, why they are working so hard to block it, particularly after the FCC has allowed huge consolidation and merger activity among NAB members.
Now, history is repeating itself yet again, as the FTC threatens to block the Whole Foods - Wild Oats merger because... it claims organic food grocery stores are a separate market from other grocery stores. Uh, right. Extra points, as in satellite radio, for claiming consumers will be irreparably harmed by a merger in a "market" that did not even exist 2 decades ago.