It's not quite as simple a just using a bit of grass. Of course I prefer not to pay landing fees at hotels too and do think if you are staying overnight with the associated revenue it's a bit rich if they charge, though in my experience few do. Lunchtimes are different - profit is usually modest.
As a land owner, increasingly vigourously interpreted and implemented health and safety legislation does mean that you have an obligation to ensure reasonable safety of your visitors and this can have costs when associated with helicopter landings.
However I suspect charging is often used to control the number of landings - too many and noise complaints and potential council efforts to restrict or prevent helicopter landings may follow.
At the end of the day these are usually private businesses and both hotel/landing site and helicopter operator can make the decisions and choices they wish to! We should bear in mind that landing fees are usually a small proportion of the cost of flying and that we do enjoy much greater freedom to land at private sites than in much of Europe.