How or when can the 250 KIAS speed limit below 10,000ft be exceeded?
Not a local thing at all!

In Australia, it is extremely common to hear, especially for aircraft on departure. Listening to LiveATC, particularly Sydney International Approach/Departure (YSSY) you will quite often hear ATC give an aircraft permission to exceed 250KIAS below 10,000ft.

Qantas 123:
Approach, QFA123, with you passing 2,300, climbing 5,000..

Sydney Approach:
QFA123, Identified, Climb FL240, cancel speed restrictions below 10,000..

There are a number of reasons this is used. The first, I have already hinted to in the example radio call above.

Departures:
It is extremely common to hear ATC cancel speed restrictions on departing aircraft. This gives better separation for aircraft departing in the same direction. For example, one of the busiest sectors in the world is Sydney - Melbourne Australia. The common SID's out of Sydney for Melbourne are the DEENA4 WOL transition and the WOL1 Departure, depending on operational runway. As a large amount of aircraft use these departures, the further the lead aircraft can get on the SID prior to the trailing aircraft becoming airborne, the better.

Arrivals:
Again, using Sydney, they receive a large amount of Regional Turbo-Prop traffic (Dash 8-Q300/Q400, Saab 340B, BE350). You will commonly hear Approach cancel speed restrictions on these aircraft on decent so they can give them more space on the approach. They land reasonably slower than a B747/A380, so the more room, the better.

Another reason is simple. These guys are operating commercial operations, hence faster is better, especially on descent. So if there is no or low traffic in the TMA, ATC will commonly release the speed restriction on aircraft to accommodate the company operations.