I guess this is probably not an issue for Sentry as it is now, but imagine some small company with a new software-as-a-service, super useful and loved everywhere, but without a well-established "name" in the market yet. Does the same answer still apply?
1. It's a big project, we are the experts (and the only real committers to the service). We also control commit access, so unless you want to maintain a complicated fork it's just not in your best interests.
2. We own the trademarks. This doesn't always help, for example this is literally a copy paste of our previous homepage: http://getsentry.cn/welcome
That said, there are companies out there who absolutely violate the 'spirit of open source' and use our code for other commercial entities. While we're not preventing them from doing that, I promise you we (and many of our users) absolutely look at that in a negative light. In a lot of ways, we're trusting that the fact that our advocates, who are developers, are also willing to do the right thing.
As you mentioned, this isn't really a threat to Sentry these days, but it was a fair concern two years ago. I think you really do have to put trust in the fact that most people aren't evil.
I have the impression that someone doing it would have a better image even with people loyal to Sentry: "well, these competitors here are not bad guys, just Sentry users like us, the only difference is that they are offering hosted Sentry for lower prices!".
(I'm not talking about the legal stuff, I'm completely ignoring it)
I don't have a concrete answer, but if it happens you just gotta work around it. Not much different than having a dozen competitors trying to do the same thing as you.
I think it's very easy to see you, as a commercial business, wouldn't want to put all of your livelihood on a project that has 150k lines of code and you know very little about. That's obviously not true for every project, but if your project is so simple there's no reason I couldn't just reimplement it anyways.
It kind of is, with Opbeat as their competitor which forked off from Sentry, but Opbeat have also kept the code that came from Sentry free.
It's also something that can limit the adoption of your product. I've worked with companies in the past have either refused to use allow GPL software, or would have a significant audit process you'd have to go through to be able to use it.
http://zeromq.org/area:licensing
Additionally, patent license provisions for using the shared code seem almost mandatory given the patent trolling we're seeing of both suppliers and users of whatever they manage to get through a patent clerk. All the FOSS licenses should try to include some protection there against beneficiaries suing derivatives.
Only with AGPL, and only if the competitors expose APGL-licensed software for interaction directly.
Suppose they've started offering Sentry-with-a-twist, how can you know they modified Sentry code? They may as well have written a separate software running in a different stack, but interfacing with Sentry and with the users and providing the twist.