I strongly suspect that the vast majority of the trespassers will not have been true enthusiasts but general members of the public who have seen the media reports of the run, including in their local papers, and who then wander down to their nearest bit of line and do what they want to get a good view. They probably otherwise never go near a railway let alone travel by train.
I hope and believe that true steam enthusiasts would never venture onto the line in the way that apparently has been happening, as the 'fraternity' is very strict with its policy on the issue.
It is a facet of any heavily-publicised event that there will be those who try to get a view by any means possible; it just so happens that the railway is an estate whose boundaries are more comprehensively protected (and with more direct and immediate consequences for breaching) than, say, roads, farmers' fields, etc.
I sincerely hope main line steam does not suffer too much as a consequence (the WCRC debacle is already doing its best there!), and that future publicity around any special trains such as today's includes clear warnings about trespass.