They expected advertisers to pay for advertising time on their channel during TNA programming, regardless of the ratings TNA pulls in, no advertisers wanted to buy a slot during their show.
It's all well and good talking about TNA being the highest rated thing on the channel, but Destination America doesn't make any money from viewing figures, they make money from selling timeslots to advertisers. If you can't get advertisers during your timeslot, then you're pretty much dead in the water.
Destination America tried the show (probably basing it off the ratings TNA were getting on Spike) but found that because of a lack of advertising, it wasn't financially viable and took the option they had in their contract to leave the deal early.
It's not douchey, it's business. There's no point getting mad at Destination America because TNA has such high running costs and a product that deters advertisers.