Why has PIE been made to necessitate us using "-pie-background:..." for defining gradients instead of just listening to "background: linear-gradient..."?

As if we didn't have enough browser-specific gradient attributes.. It makes maintaining gradient styles that much more verbose and time consuming.

There's a quick explanation here: http://css3pie.com/documentation/suppor ... background

Unfortunately this is a technical limitation. If you put something unrecognized in the real 'background' property, IE will throw away the whole rule.