It wouldn't bother or offend me to be honest i don't think its a big deal at all. As others have said maybe they couldn't afford to cater for a party and that's ok, why should the child miss out on a get together with friends because money is tight. There's no written rule to say that a party has to offer food to be considered a party, to me a group of people getting together and having fun is a party. It could be for lots of different reasons that they chose to do byo but still it wouldn't bother me and i wouldn't judge, id happily pack my child a lunch and enjoy the party.
I always cater for parties and it can cost a small fortune. On the topic of the McDonalds parties yes all the ones i had as a child and have attended the food has been paid for my the host.... HOWEVER it would not bother me to pay my own way. If it makes you unhappy don't attend but unfortunately its the kids who miss out.
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