Many countries do fine airlines for bringing passengers who don't have the right entry documentation. But that is in country's which check such things like the UK, USA etc.
Thailand immigration never does but I guess in theory they could check. But then if you showed the airline a fake onward ticket surely they would be fined anyway, so why don't they check??
Plus not all airlines to Thailand do this. I've flown to Thailand many times. Only Qatar have this policy, and in SE Asia only Air Asia and Vietnam Airlines have asked.
I just flew Qatar Business Class Venice - Doha - Bangkok and they insisted on seeing a valid flight booking leaving Thailand and the flight number too!
Qatar and Air Asia are the worst for this. Emirates never asked in January and Thai Immigration have never asked even once!
UK Specsavers is alot cheaper. I've just paid 2500baht for 2 pairs of glasses with them which are varifocals, thin and light, one sunglasses with extra blue filter, one reactive with extra digital area.
Basically every option you can get and it was still so cheap there!