Its not just Southwest but also a number of other US airlines who do it as well.
Attenpts to sue have failed as being Fat isn't regarded as a disability and Airlines have a duty to protect the safety of other passengers as well.
Can these passengers be evacuated safely in an emergency without blocking other passengers route to safety is a valid arguement.
IMHO You pay for a seat and thats it, this applies to the 70kg person as much as the 130kg person, they making inroads on your means they getting more than they paid for. Let them pay the extra.