People here should not make excuses for them because we all like railways!
This bears repeating - I think that some enthusiasts think that "the railway" should function in a bubble away from the inconveniences of the modern world - maybe we could put land mines under the trackside to deter blokes with massive cameras from getting too close next time Flying Scotsman passes by - heritage lines should be able to operate without worrying about any Health&Safety - we shouldn't have to scrap people's favourite old trains just because people expect awkward things like "level access" or "disabled toilets" in the 2020s
Or, more reasonably, landowners have certain legal responsibilities - DB are a massive organisation - they will know the requirements for the land that they own - this isn't a fine against a bunch of well meaning volunteers at a preserved railway - this is a big site that needs some protection/enforcement.
They don't have to put a million signs around it, they don't need a thousand fences, but they do need to take reasonable steps to deter people from accessing the dangerous site that they run - that's the law - DB knew the law - it looks like they've been slack in how they deal with that law.
It doesn't matter whether the people concerned were taught about railways at school or not - all dangerous sites require some protection - and the railway should and must comply with the same laws that the rest of the country comply with.
Nobody is saying that the kids were innocent, I don't think that anyone is defending them, but that still doesn't absolve DB of the responsibility to take action - and you can argue that by waging a big fine, that makes it much more cost effective for other companies to properly protect their sites (considering the "stick" if they don't comply).
Also, as an aside, and not a dig at the OP (since I've seen this on a few threads recently), but there seems to be an increase in the number of pejoratively titled threads on the Forum... I'm not a fan!