There is a minor rash of incidents of disputes in airliners over the use of devices that block the seat in front of the user from being reclined. There seems to be some confusion about the rights and wrongs of this.
This is nothing more or less than theft. Airline seats recline. If you are in coach, this may cause you some discomfort when the person in front of you reclines his seat. There are perfectly legitimate options for you to avoid this discomfort, the main one is called “business class.” The person in the seat in front of you paid for the use of that seat – including its ability to recline. If you block that ability, you are stealing a service of value, and should be treated accordingly.
If you feel that the airline seats in coach are too close together for you, then don’t fly coach. Don’t try to solve the problem by stealing someone else’s space.