It's pretty easy - the reasoning is to get these Catholics to become serious about their faith, otherwise, why do they bother being Catholic?
Tell me where in the Bible it says you must attend church... then I will tell you that the priest has a leg to stand on. You can have God-fearing/loving people that do not attend regulary
God is the only one who can get anyone to "be serious about their faith." The best that we sinners can do is continue to try, ask for God's help, and ask for forgiveness when we will inevitably fail.
Children should attend Mass. No one will dispute that, but if that isn't going to happen should we really deny them access to any religious education?
Many Catholics leave the church during college and don't find their way back until they have their own children. It may start small, like enrolling their children in religious education and continues to build. Not perfect, but better than nothing. It opens the door for serious practice of Catholicism.
To limit religious education to only those whose parents are "serious about religion" is a very self rightious practice.