First, Merlin has more chance of being seen on NBC than on BBC America. NBC is available in 99 percent of American homes. BBC America is somewhere around 60 percent.
Second, cable and satellite is not a la carte, though the companies would love that; they could charge more for each channel individually than they could when they bundle them into tiers. (That seems counter-intuitive, but it's not. Someone not watching a channel is subsidizing the people who do on their tier. If you had to pay for each channel individually, the cable companies would claim that they need to charge more because it costs more to serve each channel separately.)
Third, while there may be that sort of cultural shift in imported programming, we're at the beginning of the process. Imports simply haven't made broadcast network television here in years. Perhaps if imports take root, then we might see what you're talking about with series "migrating" to other channels in the 500-channel universe. But it's unlikely. An American network would be likely to tie up the rights for an imported series for several years, either outright or with options on first-refusal.
Basically, wamdue, this is a long way of saying, "I get what you're saying, but it doesn't work like that."
Channels here aren't "Al la carte" either but I guess tiers over there are more expensive than the "Genre" packages here, 1 package costs £18 (I think) then for the next 5 packages it's an extra £1 per package, then it's movie channels and sports that cost a lot.
But that is how it works here, mainstream channel makes a show popular. 24, House, Lost for example, then when the rights contract runs out Sky/FX/Living/whoever buy it and stick it on their channel hoping to draw more viewers/subscribers (in the case of Sky since they own the pay TV market more or less here).
