If you look closely you will see that Cryanair and Squeezy apply many of the provisions of IATA's Recommended Practice 1724 Conditions of Service. As they both have a one way pricing model there is no need for them to have provisions for sequential and complete use of a ticket.
Other airlines have different models and go far beyond the LCC in providing simple one way or round trip journeys. You can buy a single ticket on an IATA network airline and travel literally to anywhere on the planet on a single ticket, bought in one transaction in your own currency. To protect their pricing model the adopt sequential and complete use provisions.
IATA airlines are not required to apply RP1724, they can literally do as they wish subject only to government oversight wherever this still applies. I note that AF's sequential and complete use provisions are different from IATA's, although they have a similar effect. AF has chosen to have this rule because it suits their needs.
Why does RP1724 still exist, if members can do as they wish? Well, having similar or consistent provisions facilitates interlining and many network airlines want to do so.