This is apparently intentional operation. There are a few fields like this on Schedule C in TurboTax Home & Business. For such fields, the details entered in step-by-step mode are not accessible anywhere in forms mode. I reported the same issue for similar behavior of the field for self-employed health insurance payments next to line 15 and Intuit indicated to me that they have no intent to change the behavior.
The affected fields on Schedule C are editable in forms mode only if no values have previously been entered for the field in step-by-step mode. In the case of Schedule C line 1a, to be able to directly enter an amount on the form, you must go back to step-by-step mode and remove all of the entries that you made under Business Income and Expenses -> Profit or Loss from Business -> [Edit your business] -> Business Income -> General Income -> Any General Business Income?
Once those entries are removed, you will be able to directly enter on line 1a. However, making the entry directly on line 1a in forms mode voids TurboTax's accuracy guarantee with regard to any calculation error traceable to the that entry.
There are a few other fields in TurboTax that are editable only if the value has not propagated from another form in TurboTax, but those source forms themselves are linked via Data Source, and are viewable and editable in forms mode, so it isn't so much of an issue with those.
