Comment by Xywzel

YouTube's product is the commercials it targets with the collected user data, not the web or phone client. So as long as the client doesn't get in the way of showing these advertisements and collecting that user data, it should not matter to them.

In more general case, you don't pay for the client software (or web page), you pay for the service provided trough it, so it should not matter if you use the service trough different client.

Of curse, many of these new clients do provide options for getting around the data collection or make it possible to block the advertisement, and that makes the companies so strict about who makes clients and how.