When you move a parent bone, all the children move. Trying the key the counter movement is possible, but usually doesn't work very well (it may jiggle). You can change your bone hierarchy so the bones are not children. If you want them to appear like they are children sometimes (or most of the time), you can use a transform constraint. Enable it when the leg bones should move with the torso bone, disable it when they should not.

See this tip:

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The tip is a little more complex than your setup, sorry it wasn't clear enough. Here is a project showing it:
http://n4te.com/x/8519-legs.spine
Even though the hip is not a child of the torso, move the torso and the hip moves with it. Click the transform constraint and set the X and Y mixes to zero. Now when you move the torso the hip does not move with it.

I have the legs as children of the hip. If you don't have a hip, you could change the transform constraint bones to the two thigh bones. When you do this you'll find the 2 bones move to the hip bone position. This is because the constraint's offset X and Y is set to the position of the hip bone in relation to the target bone (the torso). Set the mixes to 0 so the legs move back to the unconstrained pose. Now click Match to set the offset to the first constrained bone. Set the X and Y mixes back to 100 and you'll see the bones are now both at one of the leg bones. If you need to position the constrained bones differently, use a hip bone.

For more of your understanding, delete the transform constraint, select the hip, New > transform constraint, choose the torso. Set the X and Y mixes to 100 and see what happens -- the bones move, but we probably don't want them to. Set them back to 0, click Match, and set them to 100 again. You'll see nothing happens because the transform constraint moves the hip bone to the torso bone plus the offset, and you've set the offset to the difference between the torso and hip bone. You're done, the constraint is working, move the torso to see the hip move.

Glad it helped!

It's hard to say what is happening in your animation. You could be keying the transform constraint mixes to 0 at frame 0, then translating the torso with a stepped key, so it goes from the setup pose to the key instantly. Or you could be keying the mixes with a stepped key, so it goes from the setup pose to the key instantly and the movement is because you didn't use Match as described above. Stepped keys are dashed lines in the dopesheet and graph.

Set a key (eg translation at the setup pose or mix at 100), move the timeline position ahead a few frames, then set another key (eg translation moving the torso down or mix at 0). Spine interpolates between those keys, giving you smooth movement. This is how everything in Spine is animated.

Yep, you can see the dashed line in the dopesheet showing the stepped transition.

Since your bones are moving when you change the transform constraint mix, you haven't set up the transform constraint offset like I explained above. Fix that, then you can set the mix to 0 on frame 0 so it's not active for the entire animation.

Next, set keys for translation to get translation values that change over time.