As far as I understand, there are some heuristics they rely on. Keep in mind, the on-chain fees keep changing over time.

A 10k channel with 7k fees would result in 3k usable liquidity. What if you need to close the channel? You don't have another 7k available.

I think the 100k is just a safe amount to use considering the current mempool state. And to avoid newbies losing all their money to fees because they didn't know better...

Fyi, I'm not affiliated with Mutiny, this is just my view. I hope they can explain it better.