Actually, liberals are for big government. Note: NYC is a leader in this effort, and it's pretty damn left leaning.
I don't know how it works out, but it seems silly because I personally don't know how much this ban will effect cardiovascular disease at large in the first place. There are studies of which I've read some impressive numbers, but they really don't cut the snuff for me, personally. Nutrition always has been a bit of a train wreck, with ideas changing on diet faily quickly.
On a purely rational level, it boils down to the costs of a ban versus spending on healthcare and loss of productivity as a consequence of trans fats. A quick google search has turned up diddly, so I won't even bother to estimate that. Another point along the same lines is if banning trans fats is the most efficient way to spend whatever amount of money it is, instead of other health efforts. There are a plethora of targets we could aim for in the battle against vascular disease, and I'm unsure that trans fats is the best one.
On a personal freedoms level, it's an annoying instance of a nanny state, with the inevitable question of "what are they gonna ban next"? They might as well ban sodas and juices, because they are just as nutritionally void while in moderate quantities greatly harming your health. Already, some local schools have banned sodas. I don't like being told how to eat, because with a modicum of self control I can enjoy my occasional sin and not die of a heart attack or diabetic complications, instead of the government saying I'm an idiot and I can't control myself.
Overall, I oppose banning anything really, because people should have the freedom to choose, with added weight of taking the consequences with that choice. A nanny state that holds your hand is an unattractive venture to me because it allows for decerebrate citizens.
This isn't about obesity, it's about vascular disease. A trans fat ban would not nick the problem as far as habitus is concerned, just the consequences of a diet that also leads to obesity.