Im sorry spanky but I would never ever put the safety of my kids in the bath at risk just for saving a bit of time.
The method that your tiling is being done is in my view and everyone elses on here is not safe or correct for that size and weight of tile.
A propperly fixed tile no matter the weight should stay on the wall for ever, but what you have in your home is not fixed propper and is wide open to failure, and if failure happens, whats it going to be ? , a cracked tile, or it simply comes away from the wall without notice, might be a year from now, or later, or earlier, or never, but thats the gamble your going to accept if they do not start again with a peopper bed of adhesive.
Dot and dabbing any tile is widely regarded as "cowboy'ish", even light weight ceramics, but it's so much worse being the tile you have there.
If they won't listen to you, go online and show them this thread of yours, and let them read it in full.
We're all tilers on here, and to hazzard a guess you've probably got a few hundred years of combined experiance in this thread saying the same thing, that what your
tiler is doing is wrong.
They are not going to get any credible expert proof that what they are doing is fine.
It's up to you now.