I haven't read those articles, and I'm definitely not in favor of euthanizing healthy animals. For a further caveat, I'm not even taking a stand on whether or not I think it's true, but...

There are some who believe that domestication of animals at all is equivalent to abuse. Selective breeding (which can cause unhealthy attributes along with the 'desired' ones'), tail and/or ear docks, confining them inside, etc. Removing any animal from its natural state is sometimes seen as wrong. Some people also feel that zoos are immoral and abusive.

While I don't agree with this view myself, it might permit a more sympathetic view of euthanizing pets (who are abused, genetically manipulated aberrations that should never have existed) if these ideas are part of PETA's beleif system.

Keep in mind that I do not know whetehr or not PETA holds these beliefs, but it might do a bit to explain such a practice if indeed it exists.

Cheers,

Friday