I don't think yabu.
BUT I had huge rows with my mother when PFB was a baby until she was about 4.
Mum used to collect her from nursery at about 3.00pm 3 days a week until I got in from work at 5.30pm. Was my mums choice to do this as she felt a full day until 5.30pm was too long for DD.
However, in the 2 hours she had her she would give DD her tea (understandable at that time) and then treats straight from nursery and after tea. DD soon got the jist of this and knew there was a pud coming and left most of her tea.
She also had lots of fizzy drinks in that time.
At first I would be very cross about it, to the point I got my mum in a state about what was allowed and what wasn't allowed. And she dreaded me coming in and asking what she had eaten/drank.
In the end I sat down with her and made a list of things she could have lots of like fruit and veg and chicken, things she could have some of, like sugary yoghurts and dilute drinks and things she could have occasionally like sweets and fizzy drinks.
I also pointed out that DD loves fruit and was happier with some strawberries (minus the sugar mum wanted to cover them in) than a bar of chocolate or a packet of sweets. And that he favourite drink was actually a 'grown up bottle of water from the fridge'. Because I used to fill an empty water bottle and take it into work with me.
It took a while, and even now when DD is 9 and I have relaxed a bit (shes had a can of full fat coke and non of her teeth fell out of her head for instance) Mum will still sometimes justify giving her something off the 'treat' list. She'll say 'she's had some grapes and a little orange, so I let her have a biscuit' for instance.
With your particular issue if it is just occasionally then let it go, with the explanation that it is a treat for occasional times, not as a substitute for water/milk generally. If your mum helps you out with childcare or just gives you a couple of hours to go shopping child free, or even if its just so you have a nice, easy relationship with her I found it wasn't worth arguing about or making Mum uncomfortable.
Things have changed since my mum was raising us. We were brought up with the pop man coming round, people weren't as aware about dental issues and we had more E numbers than you could shake a stick at. But we survived and have teeth and everything lol.
And those with kids who struggle to enjoy water I find that because I drink loads (still do the water bottle in the fridge thing) DD will happily drink it too. Don't get me wrong, if we do have any fizzy pop in the house for whatever reason (BBQ's/parties/christmas etc) DD would choose the fizzy stuff but if it's not there she will happily have water. Tap or otherwise!