User blog comment:FairyRaven/Fairy Tail Chapter 300: Where the Dragon's Soul Rests, Review and Commentary/@comment-904888-20120921111708

I don't think Lector and Frosch was ever made a proper member of the ST guild? At least I don't remember anywhere that would suggest that, Given Jiemma's attitude towards his guild members, I highly doubt he would actually make Frosch and Lector members. The fact lector put on his ST mark may have been a desperate move to save Sting from wrath of Jiemma (Lector's reaction, after being blasted by Jiemma, looks like as if it expected it, rather than one of surprise).

The other theory is that it is Jiemma's plan to actually enrage Sting and Lector, possibly making them stronger by making them really angry and thus bring out their true full power. Minerva's reaction would fit that it worked, and Jiemma's reaction is a surprise that he never anticipated such power.