you won't increase the demand by doing that! you just show the demand that already exists. if those people who are willing to spend bitcoin on sites like Amazon can show their interest so that Amazon can know how many are out there who want this new feature.
you can not lie about it either. because if you show fake interest and they finally add bitcoin and it were not as big as it was claimed then it would be removed and will create a big shame. for example if you make fake demands for addition of ETH you will just embarrass yourself as nobody uses that for payments.
We are the users and we have the power to request cryptocurrency payments. Amazon values its users a lot, and that's the reason they're successful. If they receive enough signals from users that want BTC/Alt-coin payment method, they will start to consider it much more seriously than now.
The only way to make cryptocurrencies mainstream is to start buying/selling stuff with them, like with fiat. Don't worry about the transaction times and fee's, the market will find a way to solve the problem if there's enough user demand.