I'm not sure this behavior is actually exploitative - you may be interpreting it incorrectly.
I usually try to switch on to whatever team I most feel like playing. Usually assault, as the very reason I play siege over battle is that I like the constant action and not waiting to respawn and fight again. Though when I'm feeling a bit lazy, I try to go for defense so I can hang out and guard the flag or whatever. I suspect that most other people switching teams do it similarly for personal preference, and not as an exploit to win.
The claim that assault wins 80% of the time.. pretty sure that one is confirmably false. If you notice, just before map change, the total victories by team - I'm not sure if this number is for the particular map or in general - you'll see that there generally is not a huge difference between assault and defense victories. Now sure, the bug whereby doors are weaker the first round does advantage attackers in the first round of some maps, and it may be that some maps are easier for one team or the other (though it is always hard to tell whether a map is intrinsically biased in this way, as so many that seem e.g. stacked in favor of defense become remarkably easy for attackers to win if they simply adopt the right strategy).
But for the most part, where you think you see the 'good' players switching themselves to the winning team, it may simply be that the winning team tends to be whichever team those players switch to. That is, more likely it is just the team with the better players that wins.
In any case, autobalance should be keeping each team roughly equal in terms of the total levels of its members and whatever other factors it considers; it's not like one team can have forty lvl 30 players, while the other side has forty lvl 10 players. It simply won't let you join a team if the other one needs you for balance - even at the beginning of the round, or if you go spectator first, or whatever. It could be that autobalance could use some improvement (I have no view on that), but it's not really designed to prevent you from switching to the team you'd prefer when balance allows.