My perspective: Being expected to add spoiler tags when drawing on the summary is a minor inconvenience for those who read the summary. Being spoiled is a major inconvenience for those who deliberately avoid the summary. As such, the weight is heavily in favour of expecting spoiler tags.
In terms of enforcement, there is the report facility so at least the number of people affected is minimised.
If people draw on the summary without making it clear where they found stuff out then people will mostly just assume it is speculation. The main issue is when people make it explicit that something is from the summary, and that should be clear cut to take action on if reported.