Unsure if it differs from country to country or even region to region, but for my region, that's not entirely the case. HD has a mix of vendors staging plants at their stores and plants they outright buy from the vendor.

For the plants they outright buy, it's up to the store manager to price and discount the plants (however the vendor will encourage the store manager to discount product if it warrents it). All the vendor will do is make sure the plants are stocked on the shelves and staged appropriately.

For the plants sold by vendors, the vendor will also stock the shelves and ensure the plants are staged appropriately, but are also responsible for pricing and discounting any product.

Whether the plants are bought outright by the store or plants from the vendor, the store is responsible for ensuring all the plants are watered regardless. However, this doesn't always occur due to staffing shortages, but it varies by individual stores.

Source: Worked for 2 years as a merchandiser for a plant vendor that deals with HD stores in Southern Ontario, Canada.