The more you use it the higher the threshold amount. The account starts at $25 now, and if you have more than two or three replenishments in a month, they will raise it and continue to do so until your normal use keep it only one or two replenishments a month.
The credit card companies get their cut (1%-2% depending on the company) for every transaction, so the state doesn't want to have to keep paying those fees several times a month.
You can try to have it lowered, but if you consistently use it every month, they wont lower it until you have a couple months of little to no use. They look at historical usage, so if you used $20 each month for the past year, then all of a sudden you used it for one month of $350, but the next month is back to $20, it may get lowered.
It raises millions of dollars the state gets to keep the interest on, but the state also has to pay the service charge for every credit card transaction. Your card gets charged $100 and $100 goes into your ezpass account. The CC company charges NYS a 1% service fee, which gets paid by the state, not your account balance.