Technically, the Black Watch tartan is an "open" tartan, as well as other military tartans, such as the Stewart Hunting (Royal Scots), Leslie (KOSB), etc. I think the "auld jocks" would get upset if one wore the Black Watch tartan and tried to impersonate a Black Watch Jock by wearing the red hackle and the rest of regimental kit -- same with any other unit. Reenactors, of course, are a different matter entirely.

The BW tartan is so popular that any attempt to restrict it's use would be utter folly -- the same with Royal Stewart, which is technically a royal tartan, and reserved for pipers in Scottish Regiments with the prefix "Royal", as in the Royal Highland Regiment, although not all wear it. Personally, it's good publicity for the regiments to allow their use, and it illustrates wear all of the civilian Scottish "kilt kit" we wear today comes from.

There are, however, tartans that are restricted, such as the Balmoral Tartan, and certain clan & family tartans where permission is needed before one can wear it from the Chief.

Cheers !

T.