We didn't have a bridal party, so I can't answer that.
But, the last wedding I was in, which we all had to travel for, the bride did pay for the dresses, since we were traveling and had to pay for tickets and accommodations.
Being in a bridal party is of course very expensive, with all of the events, travel, presents, etc. Buying the attire is such a lovely gesture and really helps out the bridal party if you can swing it.
If not, I wouldn't worry about it since so many brides are unable to do that.