Yep, that's a performance restriction (the more subscriptions you have, the more load on the database + the more email gets sent out, etc). I'll see if Mark can take a look at some of the various performance restrictions like that at some point and see if we can raise any of the limits, though!

(Generally, anything with limits like that is for database performance purposes. The code is pretty well-optimized, but larger numbers of things = more site load.)