When making an award does acceptance of the contractor's proposal mean the Government is buying the technical approach included in the proposal, or is the contractor still under obligation to meet the performance objective if their technical approach fails and may require alternatives for meeting the RFP/Contract requirements, at no additional cost?
By awarding the contract, the government has accepted the successful offeror's technical approach, so a statement is not necessary. Also, keep in mind contract types regarding the contractor's obligation to meet the performance objective. If you have a fixed price contract, the contractor has to perform IAW the scope of work. If it is cost-reimbursement (CR), then they only have to perform their best efforts; refer to FAR Clause 52.232-20 for fully funded contracts and 52.232-22 for incrementally funded contracts.
If the entire proposal is being incorporated into contracts either through page one of the contract or expressly written elsewhere in the contract, it's not generally considered best practice. The government and contractor relationship is defined by what is in the contract. The requirements that the government solicited for will obviously be included, primarily in Section C (when following the UCF); but to include a full-fledged incorporation of a proposal may prove problematic if the contractual relationship goes south and the contractor is not performing. For example, if you've incorporated their proposal and thus, their "pitch" about how they plan to meet your requirements, you could be creating the appearance that you are contracting for that "pitch" or specific methodolgy they described in their proposal, and not the requirements themselves. Simply put, it is imporant to keep in mind, a contract really should be for the 'WHAT' and not the 'HOW'. If you put the 'HOW' into a contract and it doesn't work, can the contractor really be found liable for not meeting the requirements? A much better practice, although it can be time consuming, is to incorporate very specific parts/aspects of an offeror's proposal if/where it makes sense.
So ultimately, to answer your questions, we want to use our offerors' proposals to (hopefully) find the best contractor to meet the intended performance objectives (the 'WHAT'...our requirements). But to incorporate a proposal in it's entirety and imply you are also buying a particular offeror's specific technical approach (a 'HOW'), is probably setting you up for a lengthy (and costly) dispute process if everything doesn't go as planned.