Why do forward-swept winged fighters have the main wings in the back?
These are just two canards with forward-swept wings. There is no general rule that the wing has to be in the back once it is swept forward. In both particular cases the configuration was selected to achieve the highest degree of agility possible. If you command a pitch-up in a conventional configuration, you first need to decrease lift in order to achieve your eventual goal to increase lift. In a canard, however, a pitch-up command achieves a lift increase immediately.

To prove my point that conventional configurations can also be forward-swept, here a few examples:



The first big forward swept aircraft, the Junkers 287 research airplane (picture source). The odd engine placement was the result of optimizing the cross section distribution for transsonic flight (what is today called area ruling). The white spots are pieces of sticky tape to attach wool tufts for flow visualization, and the canisters below the wing engines are rockets to shorten the take-off run. The aerodynamically shaped canister on the tripod right ahead of the vertical tail houses a film camera for tuft observation in flight.



Here is a real oddity: The Akaflieg Berlin B11 forward-swept tailless glider (picture source). It was never completed because the center of gravity turned out to be too far back. Also, the stall characteristics would have been - how should I say - interesting.

Most two-seater gliders use a slight amount of forward sweep to place the rear pilot at the center of gravity, so the plane can be flown by a single pilot without the need of adding ballast.



The HFB-320 Hansa Jet was more successful, and here the forward sweep was selected to keep the cabin free of the wing spar (picture: Eric Tammer / source: PlanePictures.Net). However this configuration was not trouble-free, either.