to think this is unacceptable
YABU because it is a free country and you do not have the right not to be offended.
and wonder why Scotland is fair game
Because there is nothing special about Scotland. Scotland is not the subject of more rude jokes than other countries. But anyway, that's beside the point. More relevantly, there is nothing special about Scottish politicians - even nationalist ones - that makes it wrong to make them the subject of satire.
You are making the classic mistake of assuming that Salmond and Sturgeon somehow represent the nation and values of Scotland. Perhaps they do. I don't know. I don't think anyone can know. I certainly don't think Steve Bell knows, which is why his cartoon is more obviously an attack on their politics.
Would you think it racist if Steve Bell made a cartoon depicting Nigel Farage as a hobbit wearing a tweed jacket and deerstalker, standing on Dover beach shaking his fist at the Dark Lord over in Brussels? If you do, you seriously need to take a lie down or a cold bath. As for me, I'd laugh at a cartoon like that, particularly if it suggested that he wanted to create a strange cultural paradise where everyone was inbred, ie, a provincial backwater.
Come to think of it, if there are many people like you, in the name of free speech, nationalist politicians need to be satirised again and again and again. Otherwise, they will become above criticism, and that cannot be right.
for racist attacks like this
It is true that being touchy is considered a Scottish stereotype. Unfortunately you have just played right up to it.
That said, I don't think it was one of Steve Bell's best efforts. I preferred this one. And ooh look! This one manages to be racist to two ethnic groups at once!