It’s in Replay’s best interest to allow for as little “down time” as possible for players who prefer tournaments to ring games. New tournaments start every fifteen minutes, with few (if any) exceptions. Since the site wouldn’t want a player to join a minute after the last set of tournaments began and sit idle for the next 14 minutes, they should allow late registration until the next set of tournaments begins 15 minutes later.
When it comes to guaranteed prize pool tournaments, generally the concern is overblown. This is only an issue for freerolls, and/or if there are very few initial registrants. For other tournaments, as the number of players increases, often the guarantee will become moot, with the prize pool generated by the entry fees swamping the initial guarantee.