<<There is a distinct possibility that TM will be the ticketing agency regardless of who the venue normally uses. This can be requested by the artist, and it appears that this holds true with this tour. U2 have requested to use TM as their ticketing agency for their North American Tour.
*Edit* What I really mean is the promoter has requested TM.>>
That is actually not true for any arena that has an exclusive ticketing contract, and virtually all of them do. Arenas have ticketing contracts with a ticketing company, and using a different ticketing company would violate that contract and they would be sued by their regular ticketing provider. In addition, an arena would not want to do that anyway because they wouldn't have (in this case) Ticketmaster's ticketing software installed in their box office and thus wouldn't be able to sell tix, print will call tix, etc.
If it is a big deal that TM be used, the promoter is likely to find another venue. This is the same reason Pearl Jam used a bunch of no name venues way back when they were fighting TM - no TM venue was permitted to sell a show using a different ticketing provider. I am certain that if U2 does play at HSBC Arena, you will see the tickets sold through Tickts.com, HSBC's ticketing company.