Who is making that statement in the bulletin on the possible cracking? Is this a known defect? It sounds like it is describing a known defect and that the cracking is abnormal rather than normal wear and tear effect. When it says the crack cannot be welded, and the part must be replaced, it is possible that there reimbursement from the factory for the repair?
I wonder what type of steel it is and if that is the reason why it cannot be welded. If the part is known to break, I wonder if it is known why it breaks.
The statement is from the factory. In view of the statement one would assume it's a known defect. If it wasn't known then obviously the factory wouldn't know about it and therefore couldn't make the statement - a bit like D1ck Cheney's "known unknowns and unknown unknowns" really.
There could well be some factory participation, it would depend on the age of the machine, hours, and previous service history. Possibly also how good a customer the owner is and how loud he shouts.
It will be nothing special as regards steel, probably a 29Mpa structural steel that you would normally repair with low-hydrogen electrodes. However what makes things difficult in this case is the square hollow tubing from which the frame is made. It's likely to be such a thin wall that it would be extremely difficult to get any thickness of weld and/or penetration on to it without blowing right through it and making things worsecin a heartbeat. As I said before, IMHO it's not that it CANNOT be weld-repaired, simply that it's unlikely that a repair would be sufficiently durable long-term to be regarded as a permanent repair. However a repair might well last long enough for the owner to come up with the coin to replace the frame, so by that criteria it would be judged as a successful repair.