Why not, they took on two dead IP's in the last 18 months too.
They probably took the decision to to try and acquire the IP back when the program had millions of viewers and was generally popular, I'm pretty sure they didn't sign a deal a week before announcing it and even the greatest speculator couldn't have predicted what has happened in the last year at AMC (multiple lawsuits, fraud accusations, storyline ruined due to language policy, massive budget cuts and a general desire to devalue the IP once all the dodgy crap they'd been up to accounting-wise was made public so it can be justified, sacking of key cast members causing the story to veer so far away from the source material that even it's leading star hasn't yet renewed his contract, probably to acumplish the same devalued state), TWD is the only show that makes AMC any money and they are facing multiple billion dollar lawsuits not just from the original show runner who was sacked some 7 years ago but the author of the source material himself), unless Merlin were very close to the inner workings of the show, I highly doubt they'd have realised what was going on behind the scenes at the time.