Although I agree that it’s not a great idea to travel close to Shabbos, as Milhouse points out, there’s no halacha against it, and the chances of actual chillul Shabbos resulting from flying one of these flights is very small.
The chances of an inconvenient Shabbos is a little bigger, but still not likely.
It’s not only people looking for a cheaper flight who take these flights, it’s often the only option, as AIY points out.
If you want to take a lesson to be more cautious when possible, fine, but I think it’s out of place to judge people for this.
I hope I didn’t spell anything wrong.