When you read through the Terms Of Transportation, you will find exceptions on amenities for weather related delays. You do not, however, find a weather exception for Alternate Transportation. They give examples (i.e. schedule change), but not exceptions in this section. If UA was operating, because the weather problem was in CLT instead of wherever UA was connecting, they should have put you on them AT YOUR REQUEST. They are certainly not going to offer it, but you can request it. This used to be called Rule 240 (it's not consistently called 240 anymore) and most of the agents will know this term and will change your ticket without question if you say you want to invoke Rule 240. Here's the text from the Terms:
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Rebooking
When a ticketed customer holding confirmed reservations on a flight will be delayed because of a schedule irregularity (whether a missed connection, flight cancellation, omission of a scheduled stop, substitution of equipment or a different class of service or schedule change), US Airways will rebook the customer on its next available flight to the customer’s ticketed destination without additional charge. If US Airways is unable to provide onward transportation, US Airways may attempt to rebook the customer on the next available flight of another airline with which US Airways has an agreement allowing the acceptance of each other’s tickets.
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My recommendation is to write a complaint letter. They did meet the first part of their Terms, by rebooking on the next US flight, but when you were going to misconnect again, they should have given you the UA option (AT YOUR REQUEST if it was still available). There is not be a set amount to which you are entitled, but you could certainly justify a request for something based on their failure to reroute you on UA when requested.

Good luck to you.