@Suiseiseki @SuperDicq net.au (australia) is a tld so mastodon is to be considered a second level in that case. Thats why.
They get to make certain rules on what they wanna their trademark to be used.
Also you do not have to have every single instance of a word trademarked to be protected as well.
Trademarks are used to protect consumers (thats the idea behind them). So consumers can make sure they are getting what they expect from brand A.
You can also infringe a trademark by being to similar to a given trademark when you are occupiying the same space in the market.