It's so simple. You walk out with a bottle and it comes up and you catch the kangaroo. So thats why you should maintain your pet kangaroo on a bottle all their life. It doesn't matter what's in that bottle, it's just getting a bottle, so its quite happy to come up and you can catch it and do with it then what you want. So its a much easier method of capture than chasing them around or darting them or netting them or running them into fences or wearing them out or waiting until they're too sick to run away.
Testicles are interesting devices that overide the brain in most cases.
Unfortunately the male kangaroo tends to view anything he sees as a mate or as a rival. Either way you're in trouble when he weighs 120 kgs and is as mean as sin. So if you're hanging on to kangaroos, get rid of the 'little bits' and the younger you get rid of them the better off you are. Sometimes they may need to be topped up with Isoflurane by mask. Consult your vet.