YANBU. I get incredibly annoyed when DSS eats the food I've bought for a snack (i.e. from a different place than the weekly 'big shop') because he honestly doesn't even tell what he's eating. I went for a shower last night for example, which takes me no more than 5 minutes. In that 5 minutes, he'd come down from his room, found something to watch on TV, and opened and finished a box of 8 luxury chocolate cookies that I'd bought me and DP (he will be at his mum's over the weekend) to share on Valentine's Day.
It wouldn't have mattered to him whether they were those biscuits or digestives. He doesn't even taste his food. He honestly wolfs it down so fast I don't know how he doesn't choke.
Me and DP put together a snack shelf for him and a little section of the fridge for what he could eat whenever he wants - otherwise he will finish all the bread (he had half a loaf, literally, with a full block of cheese in the middle of the night a few days ago), eat things I've bought for a meal etc. Honestly, one time I bought a chicken for dinner (a cooked one) and DP and I went out, and when we came back he'd eaten a chicken. An actual, full chicken. As a snack. Surely that's not normal behaviour for a 14(at the time)yo.
His snack shelf includes things like fruit, yogurts, cereal, cereal bars, and a few packs of biscuits. We are not stingy whatsoever with what we buy for him, but he will eat through everything within a day or so of us buying it (supposed to last him a week) and then just move onto other things.
I'd never ever have done that when I was 17. He doesn't contribute anything to the bills (not saying he should, just making a point), or help round the house, or even do much college work. He just sits up in his room and plays games, and comes down and eats everything that me and his dad work so hard for. I never thought I could be this resentful but it feels like an uphill struggle all the time.