Difference between a bikini top and bra?

So the other day my friends and I went shopping. After wards we decided to go to the beach it was random and didn't have swimsuits so we jumped in with our shorts and our bras. Some lady came up to us and told us we should be ashamed to be in our bras ! Like seriously ? Bra...bikini top wants the difference other than the material it's made from ? I laughed and told her well we all have different views and we're here to have fun you don't have to look at us !! Just venting lol but now I'm curious to know what you all think