The reason why many people, myself included, do not favor the "neo-jacobite" style clothing with the kilt (including the so-called Jacobite shirts) is because it's not really Highland dress, per se. It is more of a costume. It's not a style of shirt that was ever traditionally worn in the highlands. It's not an accurate historic style shirt from any period in Scotland's history. It's not a contemporary style shirt. It seems to be a made up fashion inspired more by Errol Flynn than anything else, which first appeared on the Scottish scene in the 1970s.

Another objection I have to them is that they tend to reenforce the false perception that when one wears a kilt one has to make sure that every other item of clothing you have on is "Scottish" in some way. The line of thinking goes like, "Well, I'm wearing my Scottish kilt, so I need my Scottish socks, my Scottish belt, my Scottish shirt, my Scottish jacket, my Scottish cap, etc." And while there certainly are plenty of items of clothing specifically designed to wear with the kilt (hose, jackets, bonnets, sporrans, etc.) the shirt is not one of them. There has never been a particular style of "kilt shirt" than one must wear with the kilt. Rather, kilt-wearing Scots have always simply worn whatever style of shirt in fashion at the time would be appropriate for the occasion.

So that's the major reason why many traditional kilt wearers might object to a Jacobite shirt but have no objection to a t-shirt. They are not viewing the kilt as a "Scottish costume" but rather clothing that can be worn in a variety of settings. In some of those settings a t-shirt may be very appropriate. But obviously it won't be appropriate for all occasions.

In any case, welcome to the forum, we are glad you are here!