But joint on roll cage?

Not only is it legal, but in some cases, actually required. SFI spec for area where front roll bar hoop joins upper frame rail on F/C and Dragsters chassis requires a splice with a rosette welded inner tube.
Whiitepunk- you work on nitro cars at some point (or currently)? May know you in real life- I was with JFR 2006-2011.