unless things have changed since I stopped doing sports photography, a lot of these photographers aren't paying for the lenses, full time sports photographers, could sign up for programs that Nikon And Minolta have for professionals, the program allows you to borrow lenses..
I think a lot has changed....
For starters, it depends on if you're a staffer or a "stringer" (freelancing for a publication). If you're a stringer, all your equipment is on your dime. If you're a staffer then you probably are given an equipment budget or use stuff from the publication's equipment pool. However, things in the news business (especially, the "dead tree" media world) are pretty grim right now. As circulation continues to drop almost across the board shooters are being laid off and there's a lot of pressure to cut costs and utilitze lower cost stringers. But you still don't see staffers/stringers lining up on sidelines with 300 f4's vs. 300 f2.8.
Yes, several top camera makers do have professional programs that can offer loaner equipment as well as faster equipment repairs. Nikon has NPS (Nikon Professional Services) and Canon has a CPS program. NPS and CPS also will come into town for major sporting events or news events and offer special loaner equipment as well as simple repairs/cleaning on-site. My dad used to work for Nikon and occassionally worked the on-site NPS operations at events like Indy 500's, Kentucky Derbys, NASA launches, etc. Today, I'm an NPS member.
However, NPS and CPS generally restricts loaner equipment to the replacement of personal equipment that is in for repair or special situations for irregularly occuring events. If I were to call NPS and say "Hey, I'm going to start shooting the Detroit Tigers next season and wonder if I can borrow a 500 f4 (about a $7,000 lens) for from April to September?" the answer would be "No." Their philosophy is "If you need one that often, you probably should
buy one." However, if I were to call them and say "I'll be working the Turn 1 drop gate at the Indy 500 this year and could really use a 500 f4 for Memorial Day weekend?" I'd probably get a "yes" depending on availability. Both CPS and NPS have limited loaner pools. They don't outfit the guys you see on the sidelines at NFL, NCAA, MLB, NHL, NBA, etc. games.
You are correct that Nikon and Canon view pros on sidelines are marketing tools. Canon even went so far as to use a photo of a group of photographers shooting at the Athens Olympics that showed the number of shooters using their stuff in one of their ads. But Nikon's and Canon's strategy is to use the special perks of their NPS/CPS programs as one means to keep their shooters happy. However, that strategy doesn't not extend to showering them with essentially "free" equipment.
The main point I'm trying to get across is: Buy the best glass you can afford. Photography is a "weakest link" system. Put a soft lens on a top-end camera... and you get soft images. Look beyond zoom range, consider more than just the price, etc.