They are not real FBI employees and they bought their shirts online as someone else already said, therefore they are impersonating a federal agent which is unlawful and they need to be arrested.
They are employees but they are off the clock. They need to be investigated for wearing their work uniforms while taking part in a socio-political event outside of their work hours misrepresenting the agency.
They are employees and they are on the clock being paid by taxpayers to take part in a biased political event instead of doing their jobs.
Three possibilities - none are good:
They are not real FBI employees and they bought their shirts online as someone else already said, therefore they are impersonating a federal agent which is unlawful and they need to be arrested.
They are employees but they are off the clock. They need to be investigated for wearing their work uniforms while taking part in a socio-political event outside of their work hours misrepresenting the agency.
They are employees and they are on the clock being paid by taxpayers to take part in a biased political event instead of doing their jobs.