Spying on other teams regardless of the technology has long been regarded as unsportsmanlike conduct, across all sports. This is nothing new and if anything your attitude to win using spying is against the norms.
The difference is that this was during practice sessions.
The "unsportsmanlike" aspects isn't about the difference in intelligence gathered, but avoiding the need for every team to run counter-surveillance operations.
It's a difference seeing what lineup they prepare for the match against you and specifically prepare for that, preparing for set pieces they're practicing to use against you, seeing where they're practicing to place their next penalty kick etc. - vs just general prep based on public information.
Edit: I'm sure they don't refrain from watching the opponents playing against other teams too...