The flights aren't that cheap, sure Thomsonfly's Low Cost services are cheapish but these Cancun flights are (mostly) sold as part of a package deals and so passengers have to travel with them, and may have spent a lot of money on their holiday. Going to the toilet isn't too much to ask is it!
By the sounds of things it was quite a carry on.
The spokesperson for Thomson insisted the toilets had been serviced correctly in Mexico. Why were they not emptied then?