Busking is different to tipping, and I can see someone playing music has different motivation to a blogger or someone with knowledge and skills worth sharing.

[...]

they just want to be heard mostly and blow a creative load. Most musician want an 'audience', even if they're not paying, so there is an exchange of content for 'attention'. I can see this is not applicable everywhere, but with some other 'skills'  it might be. Some people just want to be heard, and if over time their 'tips' reach a certain level, they don't mind freeloaders.

edit: with busking everyone who pays attention to you gives you 'something', maybe that's the difference

For most indie musicians, the combined upsell and pay-per-song revenue lost from reduced discovery[21] is not compensated by the impoverishing revenue sharing earned from subscriptions[22].

[21] http://www.newsweek.com/ten-indie-musicians-weigh-music-streaming-debate-355298

[22] http://pages.citebite.com/k5i1n1i6s9bch