It's easy - most of the stuff that Mesa patents isn't really patentable in the first place, and any patent attorney worth their salt can invalidate most of their stuff with a quickness. Seriously...some of the shit that they've managed to get by the patent process is just ridiculous. Like their Solo function - it's a secondary master volume that's controlled by a relay. That's it. It's not new or inventive, but it's still patented. Plenty of manufacturers use this design, but the only thing that would come out of Mesa suing them is that their patent would be thrown out.
Mesa knows this and doesn't want all of their patents overturned, so they look the other way since Bugera isn't exactly targeting their main customer base anyway.