The cheapest European fares in almost every case will require a roundtrip purchase with either a Saturday night stay (from point of arrival, which means you have to actually depart on a Friday to meet it) or a 7-day day minimum stay. If you click on the "fare rules" link when pricing out these fare, these requirements will be listed in them. Yes, this is price discrimination aimed at business travelers who typically have greater ability to spend and want to be home weekends with family. Used to be quite common on domestic routes too until the growth of the LCC's like Southwest where all roundtrips tickets are just a combination of two one-way fares (and roundtrip fares can still be found on many routes where there is no LCC competition). What you are doing is actually a prohibited ticketing practice (back-to-back ticketing to avoid minimum stay requirements). Only rarely have they actually gone after to people for this (it's typically only done in extreme cases where someone does it on a habitual basis).