well the other alternative for the restaurant is to increase prices. I agree the delivery service should not be free - but a reasonable price. If uber charges too much - then the restaurant has the choice not to use them. If enough restaurants feel the same and they stop using them then uber will have to choose to charge less.
I think the issue could be - in the current state of things - is it that the restaurants have little choice now because they can only have take away and that is likely short term so setting up their own delivery service could be too costly - not sure. To me, I would only argue against the price if it is that uber is taking advantage of the COVID-19 situation to gauge their prices.
It would be like people going out and buying all the TP and then charging people on ebay $1k a roll or something. However, if this is uber's normal charge for this service then that is simply the price of doing business and a restaurant has to decide what is best for them .
And as a consumer if you want to help a small business restaurant - you might want to go pick it up instead knowing the money will go to them.