Parler is being singled out because instead of hiring moderators as full-time employees like all the other companies you mention they decided to go the voluntee

Parler is being singled out because instead of hiring moderators as full-time employees like all the other companies you mention they decided to go the volunteer route that is less efficient. If Parler held themselves to a higher standard they wouldn't exist because most of its user base would complain about censorship go somewhere else.