They shouldn't. If it's hot enough to melt water bottles, that's too hot to play.
I don't know who's making the decisions here, but this is getting to be outrageous.
Tennis-governing bodies are not known for being particularly level-headed or logical, as we've seen from past majors (I remember when Shahar Pe'er was prevented from playing in a big non-major tournament because the idiots in charge put it in Dubai or something).