Brad Taylor points out that pig farmers are using recent controversy in New Zealand over sow crates to argue for protection against international competition and I agree that these calls have little to do with animal welfare and a lot to do with rising prices.
I have two issues with the argument about banning imports.
First I don't see how this will help the welfare of the pigs at all. All it will do is help the profits of pig framers. Why? The argument seems to be that it is imports of pork that forced local farmers to use sow stalls to remain competitive. That is, the sow stalls are the least cost method of production and thus is used to maximise profits. Why would banning imports change this? If pig framers want to maximise profits after an import ban is imposed, as they seem to want to do now, then they would use the least cost method of production post-ban in exactly the same was as they do pre-ban. That is, they will use sow stalls. As a theory of the firm man I don't like arguments that seem to assume producers will change there objective function in the face of trade restrictions. The banning of imports will have no effect pig welfare.
Secondly would the ban really help the pork industry? The issue here is the elasticity of demand for pork and bacon type products. If this is high then a increased in price resulting from the banning of imports could lead to a large fall in quantity demanded. Is it likely that the elasticity is large? At a guess yes, there are many substitutes for pig meats: fish, chicken, lamb, beef etc. A ban may not help the industry as much as some pig framers seems to think.
A better approach, if farmers really do care about animal welfare, may be product differentiation. Much like free trade coffee, "high pig welfare pork". Farmers would need to set up a credible verification system that consumers can use to identify pork that comes from farms that use framing methods that don't mistreat their pigs. Then, as is the case for fair trade coffee, consumers will self identify as those who are willing to pay more for their pork products based on farming method.
Update: Matt Nolan posts on Cruelty to pigs, willingness to pay, and intrinsic animal rights.
Update 2: Eric Crampton has been thinking about Welfare and animal welfare: NZ pork edition.