If the enemy is over reliant on battle riders, you could pin the riders when they're in battle formation, and send a task group after the tenders, then jump out of the system.
The cost of failure is too high when a fleet shows up tanks dry. Certainly there may be desperate examples where this happens, but through a combination of intelligence, planning, and luck, the attacking fleet isn't planning on showing up for a fair fight. It's showing up prepared to decisively reduce their targets.
if it was as simple as "you could pin the riders", don't you think the fleet planners may have perhaps figured that out? And make preparations to ensure that doesn't happen? Leave some riders in reserve, bring along some auxiliaries? Heck, bring the tenders in with the battle fleet, keeping them close. The defenders then get to chose to spend their precious weapon shots trying to cripple the tenders, or defeat ships that are pounding them. The tenders are OFFENSE-less, not necessarily DEFENSE-less. They'll have screens and lasers and such. Also, there's the basic tenet that it takes big ships to hurt big ships, and tenders are really big, so it's not as if you're going to send in a DestRon circling around to mop up women, children, and first aid wagons. It's going to have to be a serious amount of power, power that may better be spent trying to not get destroyed in the first place.
Hurting the tenders is a strategic option, not a tactical one. The riders are going to keep chewing you up with our without the tenders. The tenders only keep the riders from leaving, and the basic premise is that they didn't show up with the expectation that they going to be leaving anyway. You can try and kill the tail, but the teeth remain. Remember, the cost of failure is so high (i.e. destruction of the fleet), that moves are made with the highest confidence of success. Paraphrasing Heinlein, "There's nothing more expensive than a 2nd place military". They came to win. They're going to have a very good idea if they ARE going to win as soon as they show up, and can chose to simply not engage at all. Similarly the defenders can go "uh-oh" and just get out while the getting is good.
You can go after the tenders if you like, split your fleet, the riders will win the day and have a nice 2 week period to wreak all sorts of havoc on the system facilities until reinforcements show up to try and deal with them. They could head out deep outer system, waiting for recovery, maybe get lost in a local asteroid belt.
The primary reason to go for the tenders is basically a last ditch suicide ploy to try and strategically cripple the battle fleet. The system is lost already, but if you can take out the tenders, you can at least slow their advancement, make the attack that much more expensive for the attacker. But the defender will have to do that in lieu of reducing the attacking riders, and let them have their way with the defenders.