Why do so many commentators say the Sydney/Hobart race yachts cross Bass Strait?

Because they don't know their bloody geography.

The International Hydrographic Organization defines the limits of Bass Strait as follows:

On the west. The eastern limit of the Great Australian Bight [being a line from Cape Otway, Australia, to King Island and thence to Cape Grim, the northwest extreme of Tasmania].On the east. The western limit of the Tasman Sea between Gabo Island and Eddystone Point [being a line from Gabo Island (near Cape Howe, 37°30'S) to the northeast point of East Sister Island (148°E) thence along the 148th meridian to Flinders Island; beyond this Island a line running to the Eastward of the Vansittart Shoals to [Cape] Barren Island, and from Cape Barren (the easternmost point of [Cape] Barren Island) to Eddystone Point (41°S) in Tasmania].

Bass Strait is sometimes considered part of the Pacific Ocean. The IHO in its Limits of Oceans and Seas (1953) states that straits joining two seas have been allotted to one of them, but does not list such allotments. It groups Bass Strait with the South Pacific Ocean, the Great Australian Bight and the Tasman Sea. The Australian Hydrographic Service does not consider it to be part of the Southern Ocean, using the expanded Australian definition, and states that it lies within the Tasman Sea The strait between the Furneaux Islands and Tasmania is Banks Strait, a subdivision of Bass Strait.

Wiki

It's the same as people driving to Perth saying they are driving across the Nullarbor Plain. Technically they are south of the Plain just as the yachts are east of Bass Strait but it is unambiguous and convenient to expand the Plain or the Strait for such occasions.

You are blocked liar. How do you come in my questions saying anything about my English. Go out jackass.

If you answer it it means that you understand it. I dont need persons like you answering my questions, worse

contaminating my questions. Your mother tongue is not English dont try to show what you are not. Go back to your f. country and learn English you cursed.