Listening to the video they interview an employee (without showing her as she is likely only a staff member not a manager or owner) and it is immediately obvious how that letter wound up there when she says "we're a 24-hour store."
Municipalities take financial advantage of 24-hour fast food businesses to offload required emergency shelter onto them. 24-hour fast food shops are a place where vulnerable people and homeless people may end up staying for a significant amount of time. Some by choice, some by preference, others because the formal system can't or won't accommodate them. You can hire private security--and this is a thing done frequently in Toronto--but that comes at a cost, and private security is well documented to not be great at handling these situations either as they are also often poorly trained minimum wage workers. Of course, the reporter throws out red herring "whatabouts!", like disabled people, and seniors, but we know that's not what this is about.
The important fact is the staff working at these locations at 3:30 a.m. are not trained or paid to deal with homeless people who can't fit in the regular system becuase there's no space for people with mental illness, or addiction issues, and there can and have been serious or violent incidents in stores against their staff because of it. There is no 'correct' way for a business to deal with this, it's not even remotely clear how they should go about it though I would not be surprised if this is what Ottawa police advised them to do, and because it's not clear that means any windbag politician or content-starved local news reporter can turn the response of the business into their outrage of the week while themselves doing zero to actually address the issue. So you can tell them to take down the sign with Twitter hashtags (as noted in the video), but the problem which you have done zero to solve is still there for the minimum wage staff to deal with.