I'm also surprised about how aggressive some posters are! If you are so comfortable in your choices, why do you feel the need to go on the attack, eh?! Oh sorry, I forgot, it's AIBU 
"If that's the most shocking/confusing bit of parenting you've seen OP then you must live in a lovely area!"
um no. In fact I've just left an area of London that was rife with crack cocaine, and I've seen some shocking examples of parenting, which I won't repeat here as it's just too depressing.
(Am now in a lovely area!). But that doesn't mean I can't think about diet!
I'm not having a go, I was genuinely interested. For me crisps=bad (it's not just the salt, it's the saturated fat and the other crap that goes into them) and so why would you give them to a child too young to be aware of them unless you introduce them? I have been educated by this thread. Apparently I'm in the minority although I still don't get it really.
Some people are stricter than others about food, of course and that's just the way it is. My personal opinion is that once DS is older, and has found out about crisps for himself - if he wants to eat them, then a packet every so often won't do any harm IMO. But that's when he's older. I expect some people would find that a lax attitude, but I would think them food fascists 
But I am still mystified as to why you would introduce it to a child as young as 15 months?!
There is way too much salt in processed food, and it's really not good for us. I haven't salted my food for years, and when I eat processed food I can really taste it now. You do develop a tolerance for salt, and if you eat a lot of it when young (regular processed food / crisps) you'd be hard pressed not to develop a taste for it as that's how it works.
I remember when I worked as a nanny (many years ago) I added a small pinch of salt to mashed potato I made for a little girl (6) whose mum never added salt to their (mostly home-cooked) food. She instantly recognised that salt had been added and said yuck! I was surprised then that she could taste what I thought to be such a small amount, but now I've stopped eating so much salt this doesn't surprise me, I know it tastes really strong if you're not so used to it.
I the same vein, a few weeks ago, a friend made porridge for DS (then 20 months) and added salt. It's usually one of his favourites, but he refused to eat it. Again, he's not used to it so he didn't like it.
Gotta go, DS need some attention 