A lot depends on the garden, I think. In a small patio garden on a hot summer's day, wee could soon smell, not matter how sterile it is. In a big rambling garden, I can't see a huge problem.
And in this case, the little girl was only 3 after all - hardly out of nappies. Talking of which, to those who are put off by a toilet trained 3 year old weeing anywhere but the toilet, would you expect nappy changing to take place only in the bathroom too? if this person had said do you mind if I change my 2.5 year old's poo-filled nappy in the garden, is that objectionable?
I think it's a good idea to generally encourage toilet use in tinies, but really not an issue to get worked up about, certainly not for a 3 year old. But I don't know your garden, embakana, or how near the toilet was, so I don't know how I would feel in your shoes.
When my youngest son was three we had a nasty 'wee' incident at a play farm. It was a very rainy, cold day and my son was in an indoor play area there. He was very, very excited. He suddenly made a run for the open door. I ran after him, just in time to see him pull down his trousers and wee outside. I tried to restrain him but it was too late. I thought, well at least the rain will wash it away and he is weeing onto concrete, albeit near the entrance. I didn't think more of it. 10 minutes later, one of the attendants (young male) came up to me, with a dustpan and broom and bucket, and asked me to clean up the disgusting mess my son had made. I was gobsmacked, apolgised for my young sons loss of control but refused. I told him that was no way to treat paying visitors. He was on the point of asking us to leave so I asked to see the manager. The mananger duly appeared. I told my story, pointed to my son and aplogised but said I would not clean up this so called mess. The manager accepted my apology, was ok about me not cleaning up and I stayed, but it spoiled the rest of the afternoon