Side note; I'm not entirely convinced that people who do use the emergency handles for inappropriate reasons are ever pursued by the railway. At least not from my perspective on the Southeastern metro lines.
I suspect it's a mixture of two reasons.
The first being why people aren't often pursued for calling emergency services for things which aren't really an emergency but could plausibly be described as police or healthcare or urgent ish matters. In these instances we don't really want to discourage people from calling in the future when it could be a real emergency just because last time they thought their call was warranted they got slapped with a £1000 fine. Same reason we don't charge people for mountain rescue callouts when people go up Snowdon in flip flops and get frostbitten feet. We want them to call when needed, because if they don't, or they leave it too late, it can be more serious, more difficult to deal with, and more expensive.
The second being I suppose the grey area of what is 'improper use' of a passenger emergency alarm. If you're a wheelchair user and your passenger assistance doesn't turn up and the doors are about to close, is that improper use? I think it would be a bad look if nothing else to punish somebody for this. Then follows, what about non-wheelchair or assistance requiring passengers who are having their stops skipped without prior warning? Does it make a difference if it's the last service of the day and it could leave people stranded many hours from home in the middle of the night?
Pulling the emergency alarm to tell the driver a knock knock joke, clearly improper use. A train unexpectedly skipping every scheduled stop? I don't think that's so clear cut.