From wikipedia:
The GAAP matching principle. Expenses have to be matched with revenues as long as it is reasonable to do so. Expenses are recognized not when the work is performed, or when a product is produced, but when the work or the product actually makes its contribution to revenue. Only if no connection with revenue can be established, cost can be charged as expenses to the current period (e.g. office salaries and other administrative expenses). This principle allows greater evaluation of actual profitability and performance (shows how much was spent to earn revenue). Depreciation and Cost of Goods Sold are good examples of application of this principle.
Don't quite see how this violates it. To avoid any violations, all invoices would have to be produced the instant something was purchased. That's rarely the case. That's what 'acruing' an expense is all about.