There are many reasons for them to want to control the UI. One is that it is a platform where advertisements could be placed and they don't want to give that potential revenue up or let anyone else profit from placing ads there. Nevermind that consumers are revolting against the ads inserted into their television programs by their Samsung TVs; there's a potential that they could get it right.
Then there's the issue where they want to make sure that it is a human watching the cable and not a device. Time-Warner Cable pushed the "mystro" software to their cable boxes decades ago as an upgrade to their UI/UX. I was in a forced beta-test market. The way it displayed guide data interfered with the changing of channels by channel number. If you started changing the channel, the banner would pop up, the show information in the banner would be updated, and in the update the digits you'd entered would be thrown out. When raised as a serious bug, they discounted it because the workaround was just for the user to enter the channel number again and it wasn't that big of a nuisance to bother fixing. But the underlying issue was with DVRs attempting to change the channel on the cable box, which had no way to tell that the channel change failed or even that the channel change that occurred was the correct one, thus could not send a corrected channel change, and would proceed to record the wrong program off the incorrect channel. The only workaround for DVR users would be to not to change the channel on-time but skew it by a minute before. But as networks were starting to skew their own shows by a minute or two forward or back, even that could cause the channel you're *leaving* to trigger the bug. And that was conditioned on having a newer DVR that had that feature; older versions had to resort to timed manual recordings rather than ones driven by guide data, regularly titling recordings with the preceding program's information, made the guide data useless for its primary function, and network shift *still* affected them. They'd managed to design an effective captcha into their cable box to prevent usage of DVRs with their cable boxes until the CableCARD came out. So they rolled it out nationwide without a fix, because to them it *was* the fix.
No repercussions for the involuntary the beta test. Rumbles in the city government, but no effective change. They have a local monopoly.