Unless a person has vision problems or a handicap that makes filling out a slip difficult, I don't see why the clerk should do it. I agree that many store employees are not properly trained and the reason Prisoner Six gave above is a good one. Even with experience, if you're new there's a lot to learn and customers aren't patient. But sometimes the employees are just being lazy. One time I either handed a cashier at the supermarket the same card or she ran it through twice, so I had 2 tickets with the same exact 6 numbers. She said the extra ticket could not be cancelled. She probably didn't know how to do it and wouldn't call for assistance. It wasn't even busy. However, working at a convenient store like 7-11 isn't as easy as it looks. I'm not saying you need a 200 IQ or a PhD, but during busy times it can get quite hectic checking IDs for cigarettes or beer, giving directions, turning on pump #5 for $20, while keeping an eye on a possible shoplifter in the corner. They also need to make fresh coffee, check the hot dogs and throw them out after a certain time. We get a lot of tourists and I'm a mile from the beach, so the 7-11 near me often has lines. When a clerk takes his break, that one guy at the register can be doing 3 things at once. Plus the tourists who have never played the lottery in their lives suddenly decide because they're on vacation they'll spend a few bucks and have no clue what it's all about. I also hear people say "give me 2 for tonight." So what does that mean? 2 tickets for Pick-3 or Play-4 or maybe Fantasy-5. MegaMoney? How many plays? Do you want them on the same ticket? Many people see a lottery machine and think there's just "The Lottery."
BTW, one time I walked into the local 7-11 with my regular cards and the guy ran them through. Guess what? I always check my numbers before I leave and asked "where did these numbers come from?" Apparently one card was bent or didn't go through properly and he entered the numbers manually, but he entered the wrong numbers! He really was only trying to be helpful. I kept the ticket and paid the extra dollar. (I'm superstitious) Although I often play random numbers and add a quickpick or 2, in 12 years I've never changed my 2 regular sets. So this is one example of why it is always better to mark whatever you can on the card. The chances are very slim, but what if after 1,000+ draws my numbers finally came up and one was wrong! There isn't enough valium on the Earth that would help.