They're not
pets! They're "
companions" to humans!
With the growing influence of animal rights organizations in the 90s, even more politically correct jargon entered the English vernacular. While most "PC" lingo tries to purge all language that would be offensive to anyone from common dialog, these animal rights groups are inventing language so not to offend animals! Yes, there are people that believe animals can be offended.
Basically, many people believe animals cannot be "owned." Since the word "pet" has the connotation of ownership, this new catch phrase aims to eliminate misconceptions of the roles animals play on Earth.
Think that this is a non-issue? The city of Boulder, CO passed an ordinance substituting the word pet with "pet guardian" in all city laws.