Shop owner Brian Thompson, 55, advertised what are often called Kodi boxes on the front of his Middlesbrough outlet with a sign saying "Sick of paying monthly subscriptions? Free Sky, Virgin, Box Nation, Racing UK".
Teesside Crown Court heard how the losses to Sky were an estimated £200,000 in subscriptions they did not receive over the period Thompson was trading.
Judge Peter Armstrong handed him an 18-month jail sentence, suspended for two years.
Cameron Crowe, prosecuting, said streaming devices were not illegal if they were used to access free content.
He said: "If, however, they are designed, produced or adapted for the purpose of gaining unauthorised access to copyright content or subscription services - such as Sky and BT Sports - they become illegal."
Mr Crowe said a piece of software known as Kodi led the devices to be known as Kodi boxes.
He said: "Kodi boxes are not illegal per se.
"However, they allow a person to install add-ons, some of which allow unauthorised access to copyright and subscription material.
"Devices with those add-ons installed at the point of sale have become known as fully or pre-loaded devices."
Trading Standards officers made a test purchase from Thompson's Dundas shopping centre outlet in 2015 and a raid was carried out.
He moved premises after the raid and advertised on Facebook, referring to "boxes for all ya channels", "You get all the movies" and "Every film and box set ever made even ones at the cinema".
Thompson claimed the law was unclear in media interviews before he changed his pleas to guilty.
Judge Armstrong said there could be no doubt now about the legality of the fully loaded boxes.
He said: "Those who lawfully have to pay £50 a month or more on Sky or BT subscriptions, I think, are done a disservice by people like you and those who buy these devices.
"It is not a victimless crime. It has knock-on effects."
He said he was suspending Thompson's jail sentence but others in the future may not be so fortunate.
Paul Fleming, defending, said his client was a hard worker who had succeeded and failed in businesses over the years.
Thompson, of Barnaby Avenue, Middlesbrough, admitted one count of selling and one count of advertising devices "designed, produced or adapted for the purpose of enabling or facilitating the circumvention of effective technological measures".
Industry experts believe more than one million Kodi boxes have been sold.
Kieron Sharp, the chief executive of Fact (formerly the Federation Against Copyright Theft), said the people behind the sales were not "Robin Hood characters", but criminals.
He said: "Some people tend to think this is a grey area. However, it is very black and white.
"Selling pre-configured streaming devices that allow access to content you normally would have to pay for is illegal.
"Similarly, using one of these illegally pre-configured devices to stream and watch TV, sports and films without a legitimate subscription or the right permissions, is also breaking the law."
National Trading Standards said they will not hesitate to prosecute similar cases.
A spokesman said: "Consumers should not purchase these boxes and need to be aware that if they use them to access premium or subscription content then they are breaking the law."
As he left court, Thompson told reporters how, at the time, he did not believe what he did was illegal.
He said: "I accept it, I take it on the chin, that's it."