To install an Extension that has been monetised, the broadcaster must be a partner or an affiliate. The reasoning behind this is that they already have a means to by which they receive payouts. Developers earn money from in-Extensions purchases differently from partners and affiliates. If you are a developer and broadcast and want to earn money as both a developer and a broadcaster (so many uses of the word “and”), you need to be a partner or an affiliate. Being an Extensions Developer is not a backdoor to becoming a partner or affiliate though!
Hopefully this helps!