Whether this is fraud or not, depends on many things like the question which law applies to this case, what intent you had in the moment you presented the ticket to the agent, etc.

To just say it is fraud or it is not fraud without further information does not make sense.

It does sound from your post like you were aware that you were breaking the rules, so I guess you cannot be too surprised if they confiscate the remainder of the ticket.

Whether it was fraud or not is another question, but you apparently willingly broke the terms of the contract you had with the airline, so (at a first glance) confiscation of the ticket by the airline does not seem inappropriate (I would assume that they do reserve that right somewhere in their standard terms and conditions for the case that you break the rules anyway).