I think there are a couple of reasons for this.
So, firstly, new rules say that supply staff can be used to cover for striking teachers- this is the first time this has been the case. As you can imagine, there was high demand from some schools for supply today, and not all supply staff will cross a picket line, so part of the reason for this advice was to stop schools being able to book supply teachers in advance.
Secondly, some heads would put pressure on staff who indicated they were striking not to strike. This isn't legal, but you know it would happen. In these cases, it's sensible just to say you haven't decided yet or you aren't going to say.
Thirdly, some schools have (potentially illegally) indicated to parents which teachers are striking- obviously no-one will say before the next strike day now!
And finally, it is the right of people not to make up their mind until the day. We had new union members join us on strike today. They may not have known a week ago what they were going to do. People can also choose not to cross a picket line at their place of work.
The NEU did let each school know how many members were in that school 2 weeks ago, so heads may have had some basis for planning.