Unfortunately, and contrary to what you imply, this actually seems to be the rule in alliances: most airlines do not offer free access to contract lounges to elite pax of other alliance airlines. Even if it may be characterised as a race to the bottom, TP is following the rule of full service airlines.
As a SkyTeam ElitePlus, I've had various SkyTeam carriers provide me with contract lounge access even when flying them in economy class and being an OAL SkyTeam ElitePlus.
As a Star Gold, I've had various Star Alliance carriers provide me with contract lounge access even when flying them in economy class and being an OAL Star Gold.
As a Oneworld Sapphire/Emerald, I've had various Oneworld carriers provide me with contract lounge access even when flying them in economy class and being an OAL Sapphire/Emerald.