Don't know how Suzuki could have made their swingarms with out heat treating them.
The extruded arms are usually bent inwards at the front as the pivot is narrower than the axle end. If you try to bend 6061 while it is in T6 state then it will most likely crack at the bend.
The banana arms that I build for my HL's have the extruded sections annealed to T0 so they can be bent then taken back to T6 after welding is finished.
Quite often though there is so much welding in an arm that the heat from the weld will take it out of the T6 state anyway.
To answer Tomas question, if there was only light heat that went into the arm then it will probably be allright. The heat effected zone of the weld won't be in the T6 state but if it is only in a small area and not through the whole section of the arm then it will only be a minor compromise. You can usually fill bolt holes and small cracks without drama.
