It usually happens when the batteries stay in the unit for a longer time. AA will leak as well. It's often hard to tell how old some batteries are. They could have been in the shop for 2 years before they got sold. Duracell does a lot of advertising, but they start leaking as well. Due to the fact they have a higher capacity, they will be used longer and have a higher possibility of leaking.
For things like a scope, I would personally use a power adapter. They usually draw 2 much current to work well on batteries.
For some equipment, the fact it can work on batteries is more a marketing gimmick than a really usefull feature.