arc you said it yourself, when you “buy” the tickets you are supporting that organization. That doesn’t mean if you win, your intention is to continue that support with the your 10%. You could possibly choose to give part of it, half of it, all of it or none of it. It is not up to the organization to assume that. You might be supporting various institutions or have hakoras hatov to many. You might wish to support a local Rav, or a poor family that you wanted to help but couldn’t before. Your child might be very behind in tuition payment and they might be threatening to kick your grandchild out of yeshiva, that donation might be a gift from Hashem to help not only that child but all your children with tuition and/or mortgage payments.
I am sorry but I just don’t agree with you, it is utter chutzpah to make the assumption that anyone would WANT you to automatically deduct the maaser for yourself. And it is illegal. The organization should remit the entire amount and include an envelope in case they wish to make a contribution.