You should always hire the best candidate which you think will work out but that month allow you to set the right phase, this is a month where you both give the best to make that relationship work - orientation, pairing, documentation, short meetings to discuss potential roadblocks or issues.
All startups usually do this - I just consider it's a good practice to say it out loud that there's a period where we both find out if we want to keep working together.
It's important to remember that the "companies prostrating themselves at your front door offering you bonuses and perks and lavishing dining you" is an American Tech Hub phenomenon. While I doubt UK coders are doing badly, I doubt they are as in-demand as they would be in SF or NYC.
That said, I'm "old" (36), and have family responsibilities. If I were a naive recent grad I might have entertained doing this.
Trying out at a company for a month isn't really that bad when the demand is so intense you can literally line up 3-4 competing offers in under a week if things don't work out. There is sadly no place in Canada where the demand is that high.