A very valid point to be honest.
I do see where you are coming from but the buckets themselves have a fairly finite life and get thrown out. Generally we leave the adhesive to dry in buckets over night and knock them out on the side of a skip the next day. We usually get around 5 or 6 mixes out of a bucket this way before we end up holing the flexible bucket (maybe we should buy the more expensive ones rather than the £5 ones). In addition, we need to have separate buckets for mixing adhesives and grouts and plastering really to avoid contamination. We end up with an awful lot of buckets really. In addition, cleaning out buckets uses quite a lot of water which is very environmentally unfriendly too!
With liners you can get away with one bucket per tiler to be honest, covering grouting, plastering, tiling, whatever you want. A 2mm liner on each mix will massively extend the life of the buckets.
I think that the liners (which would be made from recycled plastic) would both extend the life of the more expensive buckets and be less wasteful than throwing these buckets when they get holes in them.