I think that Turkish Airlines can be excluded since Erdogan said he will defy US sanctions on Iran and Turkish Energy Minister Fatih Donmez said that Turkey has a waiver on US sanctions due to energy dependence:
Washington says it will temporarily allow eight “jurisdictions” to
keep importing Iranian oil when the sanctions come back into force on
“Just a while ago ministry officials brought the news that the U.S.
Secretary of State made a statement on limiting or lifting of
sanctions for eight countries including Turkey,” Donmez told
NATO member Turkey depends heavily on imports to meet its energy needs
and neighboring Iran has been one of its main sources of oil because
of its proximity, the quality of its crude and favorable price
For Lufthansa and its subsidiary Austrian airlines, this article argues about German companies being prepared to continue doing business in Iran despite US sanctions:
The US ambassador to Germany called on companies to leave Iran
immediately. But German companies, which have a long history in the
country, have been preparing for this moment for months, effectively
cutting the US out of any Iran-related business.
“For some time now, companies doing business in Iran have been trying
to find substitutes for any US-related aspects of their supply
chains.” For example, European companies in Iran do not work with
dollars at all; they prefer euros or perhaps dirhams. “Any US staff
members are excluded from Iran-related business,” Mr. Tockuss noted.
“There have been a wide variety of preparations and many companies
have been building a fence around their Iran business.”
Also it might be related to the investments performed in order to increase the traffic following the raise of sanctions which would result in net loss if all flights are cancelled:
German airliner Lufthansa and its subsidiary Austrian airlines will
offer new flights to Iran amid a growing demand as a result of the
business and tourist boom in that country, according to the German
aviation website aerotelegraph.
Lufthansa will now fly also from Munich to Tehran and switch to larger
planes for its Iran flights from Frankfurt to Tehran effective summer