I imagine if it doesn’t make sense to you already, I won’t be able to help you understand, but here goes anyway.
A company pays a lot of money to have a service developed and hosted, then offers a paid teir, and an ad supported teir. If you neither pay, nor watch ads, yet still use the service, then the company is incurring fees due to your usage while receiving no revenue for providing the service.
Like, YouTube videos are great. Practically a utility at this point. However YouTube, the platform, is a company not a charity; if you cost them money, while providing $0 revenue, you’re stealing their money.
Personally I don’t think theft is justified outside extreme desparation (stealing food, basically), and given that YouTube gives you an option that does not require giving them money I find it hard to justify stealing bandwidth.