Cherished I think I know where you are coming from. You feel that there is plenty of time for babies to eat rubbish food once they grow up so why start so young. To an extent I agree with you - I remember my SIL giving my nephew milo at age 2 or so and thinking it was a bit young. I think where some of the objection is coming from is that the older your child is (and equally the number of children you have) the less you tend to sweat the small stuff. I know you don't think junk food is small stuff - perhaps you never will - but many off us start out with ideas and ideals that over time we let slide or totally discard. Before I had kids I would never use the electronic babysitter that is television. And what do you think my four-year-old is obsessed with right now? The very same four-year old who is also incredibly bright and active and reads books like others play with nintendos? He is obsessed with "The Ocotonauts". So TV is suddenly a big part of his, and hence his brother's, and his 5 month old sister's(!) day. For now. Because he will change. So maybe, just maybe, those mothers are eating maccas now because their kid is going thru an "I eat nothing else" phase. Maybe they have older kids with them and the baby may as well join in. Maybe, as others have said, they were strict with their older kids and realised that getting hung up on one thing or the other made no difference in the long run.
Yes, maybe they are setting their kids up for obesity because maccas is a regular thing, but equally maybe its not.
Did any of the mothers you see have older kids