Two reasons:
1) even if you happen to like cats, it does not mean everyone likes them;
2) animal protection, not only regarding birds, squirrels and what have you, but also for the cats' own sake. They may be "resourceful" and "independent" but not even nearly as much so as the really resourceful and independent animals at these latitudes, ie the species that belong to the Finnish nature; especially so when temperatures drop considerably below zero. Callous pet owners (yep, they do exist) would easily dodge responsibility - lock the animal out at -20 degrees, and then say "it needs to roam free - it WANTED to go out".
Also, it may not be nice if (and when) your pet cat is accidentally run over by a car; the car drivers do not necessarily like the experience of flattening someone's pet either.
I don't think 1) is a valid reason in itself. I don't like women in high heels (they make an irritating noise and clog up traffic flow on the pavement because they can't walk properly), but wouldn't for a moment suggest that there should be a law against them. If unsupervised cats posed a
significant danger or
major inconvenience to many people (as unsupervised dogs would) then that would be a different matter, but they don't really.
As far as 2) goes, there are two different issues here. There may be a case in Finland for cat control for the sake of bird or other animal protection for all I know, but I'd have to see the evidence before I was convinced. Given the relatively small extent of urban areas in Finland it's hard to believe that there would be a detectable impact on wild bird population levels from domestic cats. There could potentially be a small conflict between the interests of people who wanted cats around and easily visible in their neighborhoods and those who wanted easily visible birds, but again this is by no means a given. The other point, i.e. protecting cats for their own sake, assumes that your average cat is better off in a relatively risk free environment in which it is deprived of its freedom and natural inclinations than it is being in a riskier environment where it is free to roam. From my knowledge of cats I certainly don't think this is the case. Same goes for intentional cruelty - the suffering of the small minority of cats who are forcibly locked out in -20C temperatures has to be balanced against the smaller suffering of the nonetheless far greater number of cats who would otherwise be deprived of their freedom. Re: getting run over - this is a big risk, sure, but one that most owners accept (and more an argument in favour of banning cars from towns than cats I think, to add to all the other ones). As far as psychological trauma to drivers is concerned, to the extent that this is any greater than that associated with running over a wild rabbit or bird, it is presumably due to the the animal being a pet. As long as cat owners accept the risk and said flattenings are accidental, then it would be bizarre for cats to be banned in order to assuage the misplaced guilt of car drivers.
I am very aware that due to my staggeringly bad Finnish I have only got the vague jist of some of those 16 reasons on the page you posted to, EP, and that some of them may contradict what I am saying... if so, I stand to be corrected!