Question: if I buy a bottle of water after security, who has checked that these bottles of water are safe?
What on earth do you mean? Safe - the bottle or the water? They are security screened and come form the retailers/suppliers. If they didn't contain water it would become apparent when the first person who bought one took a drink. If some contained chemical then the staff would have to be complicit in arranging for these to find their way into the right( wrong) hands.
The fact that they could be used as a vessel to contain a greater volume of liquid remains a potential threat - but the security teams obviuolsy assume that they will intercept and dangerous chemicals under the 100ml rule which would prevent multiple terrorsits amalgamating their chemical airside.