Whoa why would they disband that's mean to be mean with no benefits to it... quite forced author...
It's not forced. If that's rule was not set in place then when one party loses they'll just try to get stronger and demand that competition another time and can repeat the process which will cost a lot of resources and time, which the empire does not have because they're preparing to fight a valiant.
There's also the fact that they are trying to limit parties from becoming clans and I'm going to assume that other parties have applied to become clients and have been rejected. However if it got out the fact that you can become a clan if you just defeated another party in a raid, then numerous parties will come and try to test their fate, which defeated the purpose of rejecting the parties in the first place to stop the creation of clans and focusing on current clans and their fighting power.
And therefore it only makes sense that a clause be put in that contract so parties are deterred from wanting to become a clan because they may not get to act as a party anymore and as seen in previous chapters; it's very difficult to form parties because potential party members are scarce in the city.