What if my fighter doesn't want to be distracted by the ruffian's ally? Note, not all thieves are pc's.
I'm sure you don't want to be stabbed by the ruffian either, but you don't get a choice. You are fighting people that intend you harm, they are not going to worry about what you want. This is a different relationship than with your allies.
And, if my fighter lets your thief hit better, why don't I help anyone else?
You are not giving the thief the ability to Sneak Attack. That is his own skill. You aren't giving him anything, the thief is taking advantage you fighting the same opponent.
Note, killing a target in no way takes away player agency. Just because the dragon breathed on you is not a loss of player agency for you. Otherwise all combat is a loss of player agency.
Here is a dictionary definition of agency that I think applies: the capacity, condition, or state of acting or of exerting power.
If your character just takes damage, no, you haven't lost agency. If your character is killed you can not act or exert power with that character. Sounds like the loss of agency to me. But it is okay to lose agency in combat. Each side is trying to harm the other. That's the whole point of combat.
And, if you are helping my rogue stab someone why can't I help you stab someone? Why is it okay that I force you to distract my target, but it's not okay for me to distract a target for you - a typical warlord power- and let you hit better?
Anyone can help anyone else. It's called the Help action. It's just that the rogue is better at taking advantage of fighting with others.
Sneak Attack isn't something that anyone else gives the rogue, it is something the rogue is capable of because he trained himself to be good at that style of fighting.