These don't break fare rules, they break ticketing rule restrictions. These are two different subjects. However, OP is in fact talking about ticketing rule restrictions and not fare rule fare combination restrictions (restrictions on combining multiple fares on a single ticket). You can't break fare rule fare combination restrictions because the system will prevent you from purchasing such combinations on a single ticket in the first place. Note that just because combining certain fares on a single ticket is prohibited by the fare rules does not mean you are breaking ticketing rules by buying such fares on multiple tickets.

The classic example is nesting two separate roundtrip tickets in order to save money by flying out of a cheaper gateway. The system will generally prevent you from combining these fares on a single ticket due to fare rule combination restrictions and you will have to buy them as separate tickets. However, this is generally not a ticketing rule violation unless, as you noted above, you are nesting them to get around minimum stay/Saturday night stay restrictions in the fare rules of roundtrip tickets (which generally requires the nested roundtrip ticket to be booked in the reverse direction). This is also known as back-to-back ticketing and is mentioned in the prohibited ticketing practices (see below).

Delta ticketing prohibitions are documented here -- https://www.delta.com/us/en/booking-...s-restrictions
It is also documented in the Delta Contract of Carriage.