That isn't a catch. A catch would be some sneaky extra bit thrown in that likely required something of the or made some ineligible or whatever. They're were never going to give a reason to begin with and whether they tell anyone or not doesn't change the fact that they didn't have to give any free days but did. And they require nothing of the as they're automatically claimed.

Surveys, sales, reader , website traffic, critical response, manga volume sales, etc. This series in particular is a digital only series, so it's pretty easy to see how many people read the first chapter and then went on to the second. If people aren't reading it much at all or there's a significant drop-off after the first chapter or two, it'll probably get cancelled. I don't know specific data regarding this series, but it must have just had very little readership.

"A Popular Shonen Jump Series Is About to End Way Before Its Time"

Seems like it's ending exactly at its time. It might suck for the small contingent of fans, but the reality is.. it wasn't popular.