Because it's short-sighted and might well have the opposite effect: drive people away from the site all together.
Not to mention it's very dishonorable to update your software so that previously purchased products become less useful. Imagine if Microsoft released a service pack for all versions of windows that made them run 10% slower and then charged to have it patched. It would be a scandel and rightly so and would forever dent their reputation and market share. Such behaviour is bordering on extortion, which is illegal with good reason.
Maximising profits is only acceptable in certain ways and only wise if it doesn't drive away existing customers and/or deter new ones.
I honestly expect such behaviour to be detrimental to their profits in the long run. Businesses that try to screw existing customers for as much as they can get don't last long. What will they try next when the revenue stream from this scheme dries up? And then the next one and the next and the next. Each making less and less.
If they spent more time opening up the other courses and giving driving ranges and putting greens and multiple saved games then there could be a case for charging more.
As it is, it looks like an unfinished piece of work and WGT seem to be in no hurry to finish it before trying to charge as much as they can for it. Successful businesses do not work that way. It's the business plan of a fly-by-night operation intending to fold after embezzling as much as they can first.
I just hope WGT realise this before it's too late and sack the manager that keeps coming up with these ideas and employ someone who knows what their doing. It has the makings of a great site and a great game. It would be a shame to lose it.