I see 2 separate issues here:

1) Putting things under your own seat, instead of under the seat in front of you, is obviously incorrect, rude, etc. This is really the passenger with that seat's issue.

2) The "nothing under the seat in front of the exit row" isn't a standard worldwide; I've only heard of it in Britain, but it may be true elsewhere. Certainly in the US that underseat stowage is used.

It isn't as ironclad as some think -- my wife and I (from the USA) flew British Midland in 2001, got seated in the exit row and put our backpacks under the seat in front. Another passenger got very agitated that we absolutely couldn't put anything there, so I asked the FA (would have been willing to put them overhead if required, of course, but we prefer to have them accessible if possible). She looked at them and said as long as they were shoved all the way back they were okay. Is the written rule that nothing may be there, or just that the actual walkway be kept clear, with more vigilance to long bags in that row than others?

That doesn't excuse using another passenger's space, of course, but it may well be that a small handbag etc is allowed to be placed there.