Developers cannot detect whether a player leaves a like on the game or not, so they attempt to trick the player into believing otherwise.
Basically, they tell you to like, favorite and join the group, then they use code to check if you’re in the group or not. Since favorited games of every user are public information, I’m sure there’s an easy way to check this as well. The only thing they cannot check, are likes, for obvious reasons.
You’re not supposed to give players bonuses for liking the game at all, for two reasons:
1: The like system is intended to be a rating of the game, not just another cash-grab feature for people’s projects to misrepresent the quality of their game
2: It’s dishonest to claim liking the game will give perks, because this is completely false
You’re allowed to ask players to join a group for perks however, as this is the appropriate channel of self-promotion on Roblox.