This is a good point and often overlooked with high impedance passive probes.
The bandwidth of the probe is specified assuming a 25 ohm source impedance from
a terminated 50 ohm test fixture which is not realistic in most cases. Passive
probe bandwidth will be limited by capacitance at the probe tip and the source
impedance no matter what the bandwidth specification says.

I wish there were low impedance passive probes like this available
inexpensively. Tektronix recommended the P6056 (x10 500 ohms) and P6057 (x100
5000 ohms) but they are not all that common and are often expensive even used.

PMK makes some but I have no idea where to buy them and they are not
inexpensive:

http://www.pmk.de/en/products/pml_low_z_probes